An "anti-terrorist operation" is under way in Ukraine's eastern Donetsk region against pro-Russia protesters, Kiev declared on Tuesday, with armored personnel carriers seen on the road in the restive area.
Twenty-four hours after a Ukrainian ultimatum expired for the protesters to lay down their arms, acting President Oleksandr Turchynov told Parliament the operation would be "conducted step by step, responsibly, cautiously."
"The aim of these actions is to protect the citizens of Ukraine, to stop terror, to stop criminality, to stop attempts to tear Ukraine to pieces," he said.
Tensions have soared in recent days in eastern Ukraine, where pro-Russian separatists have seized government and police buildings in as many as 10 towns and cities. Russia declared its neighbor was on the brink of a civil war.
A Monday morning deadline set by Turchynov for protesters to disarm or face action by Ukraine's armed forces passed without any sign it was heeded. But Ukrainian forces now appear to be on the move en masse toward the region.
A spokesman for Ukraine's National Security and Defense Council, Evgen Rojenyuk, confirmed a National Guard battalion made up of 350 troops was sent to the Donetsk region from Kiev on Tuesday morning.
The deployment was also confirmed by the deputy head of Ukraine's Internal Affairs Ministry, Mykola Velychkovych, though he would not confirm the numbers involved, where the battalion was headed, or what orders it would be given. He said the ministry would decide on a plan of action once the troops arrive in eastern Ukraine.
The National Guard troops -- many of them former street protesters who signed up after the interim government took power in February -- have had only a few weeks of training.
A CNN team in eastern Ukraine encountered a large Ukrainian military column traveling on roads leading from the city of Donetsk toward other towns in the region.
The column included more than 20 armored personnel carriers, along with support vehicles, and a helicopter circled overhead. The troops, who were carrying the Ukrainian flag, appeared to be headed north and east, although their precise destination was unclear.
The pro-Russian mayor of Slaviansk claimed Tuesday that a Ukrainian military convoy including troops and armored vehicles had now surrounded the town, Russian state news agency ITAR-Tass reported.
"If they try to move in, we will have to stop them," it quoted him saying.
Pro-Russian protesters seized a police building over the weekend in Slaviansk, some 100 miles from the border with Russia.
From a hill overlooking the town, a CNN team saw no obvious signs of a large-scale operation, such as military helicopters or planes.
Earlier, police in Kramatorsk, a town to the north of the Donetsk region, appeared to have taken back control of a police headquarters from pro-Russian activists. The City Council building was still occupied, however.
Turchynov's office later added Ukrainian special forces were in control of an airfield in the town -- where witnesses had earlier said they had heard planes flying overhead and gunfire.
In other signs of movement, amateur video footage posted to YouTube on Tuesday purportedly showed a Ukrainian military column, made up of trucks followed by tanks, that is believed to be pushing through the Donetsk region. CNN cannot independently verify the authenticity of the video.
Another unverified amateur video clip posted on YouTube shows a Ukrainian military tank, also believed to be in the Donetsk region. People who appear to be civilians approach the lone tank to ask its crew, "Why have you come? ... Are you here to kill us?" No violence occurs.
Amid the unfolding crisis, Ukraine's most senior security and defense officials are to meet for a closed session with all members of Parliament on Wednesday morning, Turchynov said.
Russian Prime Minister Dmitry Medvedev gave a gloomy assessment.
"Blood has been shed in Ukraine again. The country is on the threshold of a civil war," he said on his Facebook page, according to state news agency RIA Novosti.
NATO chief urges Russia to help
EU defense ministers and NATO Secretary General Anders Fogh Rasmussen met Tuesday in Luxembourg, with the crisis in Ukraine high on the agenda.
As he arrived for the talks, Rasmussen told reporters he was "deeply concerned" by the latest developments in Ukraine, citing continued violence by small groups of separatists and Russia's continued military pressure on Ukraine's border.
"I call on Russia to de-escalate the crisis, to pull back its troops from Ukraine's borders, to stop destabilizing the situation in Ukraine and make clear that it doesn't support the violent actions of pro-Russian separatists," he said.
"Russia should stop being part of the problem
and start being part of the solution."
He said NATO and the European Union are not discussing military options, and that they support a diplomatic and political solution to the crisis.
"However, we are focused militarily on strengthening defense of our allies. That's our core task," he said.
A U.N. human rights report released Tuesday on the situation in Ukraine, including the Crimea region -- which was annexed by Russia last month -- cited an urgent need to counter "misinformation, propaganda and incitement to hatred" in the country to avoid the further escalation of tensions.
"Facts on the ground need to be established to help reduce the risk of radically different narratives being exploited for political ends," said U.N. High Commissioner for Human Rights Navi Pillay.
Based on information gathered by U.N. human rights monitors since March 15, the report draws concerning parallels between what happened in Crimea and events unfolding in eastern Ukraine now.
"In eastern Ukraine, where a large ethnic Russian minority resides, the situation remains particularly tense," the report said, as it urged steps to build confidence between the government in Kiev and people living there.
The U.N. agency also cited allegations that some Russians have been involved in stirring up the unrest.
"It is widely assessed that while there were some attacks against the ethnic Russian community, these were neither systematic nor widespread," it said. "There are also allegations that some participants in the protests and clashes in eastern Ukraine were not from the region, and that some had come from the Russian Federation."
Russia has said it reserves the right to intervene to protect the rights of ethnic Russians in eastern Ukraine.
In a sign of the divisions roiling the country, YouTube footage posted online appeared to show two openly pro-Russian presidential candidates, who have demonstrated support for separatists, being antagonized by crowds in separate incidents in Kiev on Monday night.
In one video, Mikhail Dobkin, a presidential candidate for the Party of Regions, the party of ousted President Viktor Yanukovych, had flour and green liquid thrown at him as he addressed crowds.
Another clip showed Oleg Tsarov, an independent candidate who was expelled from the Party of Regions, speaking to people without a shirt as he apparently sought to show the bruising he'd received from an aggressive crowd.
In the third video, Tsarov tries to leave a TV station, where crowds shout "Shame! Shame! Shame!" His supporters and protesters end up in a brief scuffle.
CNN has not been able independently to verify the amateur video clips.
Obama urges Putin to use influence
Also Monday, U.S. President Barack Obama and Russian President Vladimir Putin spoke about the worsening crisis. The two leaders last spoke March 28.
According to the White House, Obama urged Putin to withdraw Russian troops from Ukraine's border and to use his influence to persuade armed pro-Russian groups to leave the buildings they have seized.
"The President noted Russia's growing political and economic isolation as a result of its actions in Ukraine and made clear that the costs Russia already has incurred will increase if those actions persist," the White House said.
According to a Russian statement, Putin stressed the protests "are the result of the Kiev authorities' unwillingness and inability to take into account the interests of the Russian and Russian-speaking population" and called on Obama to help prevent the use of force.
"In response to the U.S. President's expressed concern about Russia's alleged interference in southeastern Ukraine, the President of Russia noted that such speculations are based on inaccurate information," it read.
European Union foreign ministers on Monday agreed to expand the list of those subject to sanctions and took further steps to help Ukraine economically, as Kiev's interim government seeks closer ties to the West.
"Any further action aimed at destabilizing Ukraine must stop. It is crucial that Russia and Ukraine engage in a meaningful dialogue to find a political solution," said EU foreign policy chief Catherine Ashton.
Leaders from Ukraine, Russia, the United States and the European Union are due to meet Thursday in Geneva, Switzerland, to discuss the crisis.
Pretext for intervention?
The unrest is the latest in a series of events ratcheting up tensions between Ukraine and Russia, which Kiev accuses of fomenting trouble in Russian-speaking eastern Ukraine.
After then-Ukrainian leader Yanukovych backed out of a deal with the European Union in November in favor of closer ties with Russia, he was forced from office in February, the result of months of protests in Kiev.
Distrust among the population in eastern Ukraine, the base of Yanukovych's power, grew as the new national government shifted rapidly in a pro-Western direction. A short time later, pro-Russian elements occupied the Black Sea peninsula of Crimea, which Russia annexed in March.
Since then, pro-Russian protesters have taken to the streets in eastern Ukraine and in some cases stormed and occupied buildings.
Kiev's fragile new government and the West accuse Russia of destabilizing the region as a pretext potentially to send in troops to protect the local Russian-speaking population.
NATO says Russian armed forces are massing on Ukraine's eastern border, while Moscow says they are merely carrying out military exercises.