Armoured vehicles bearing Russian flags rolled through a flashpoint town in eastern Ukraine Wednesday, a day after Russian President Vladimir Putin warned the country was on the brink of civil war.
Tensions continued to mount ahead of high-level diplomatic talks on the Ukraine crisis in Geneva on Thursday after Kiev sent in troops to oust pro-Moscow separatists from the east.
An AFP reporter in the town of Slavyansk saw at least six APCs, some flying Russian flags, carrying dozens of armed men through the town.
Russian media said Ukrainian troops in the vehicles had switched sides to join the separatists but the Ukrainian army told AFP that it had no reports that any of its equipment had been seized.
Elsewhere, pro-Russian gunmen stormed the mayor's office in the regional capital of Donetsk, according to an AFP reporter at the scene.
As the situation on the ground appeared to escalate, the authorities in Kiev ratcheted up the verbal attack on Russia, with Prime Minister Arseniy Yatsenyuk accusing Moscow of trying to build "a new Berlin wall".
Russian commanders in the separatist east had issued pro-Kremlin militants with "shoot-to-kill" orders, the Ukrainian Security Service (SBU) said in a statement.
Yatsenyuk demanded Moscow halt its alleged support for the separatists.
"There is only one directive for the Ukrainian foreign ministry -- the Russian government has to immediately withdraw its commando groups, condemn the terrorists and demand they leave the installations," he said.
Nevertheless, he said Kiev remained committed to Thursday's crunch talks scheduled in Geneva between the top diplomats of Russia, the European Union, the United States and Ukraine.
On Tuesday, authorities in Kiev launched what they called an "anti-terrorist operation", sending tanks towards Slavyansk -- which remains effectively under the control of pro-Russian gunmen -- in a high-risk strategy sharply condemned by the Kremlin but supported in Washington.
The 20 tanks and armoured personnel carriers sent to Slavyansk were the most forceful response yet by the Western-backed government in Kiev to the pro-Kremlin militants' occupation of state buildings in nearly 10 cities across Ukraine's industrial heartland.
But the move drew a sharp response from Putin in a telephone conversation with German Chancellor Angela Merkel.
"The Russian president remarked that the sharp escalation of the conflict has placed the country, in effect, on the verge of civil war," the Kremlin said in a statement.
But both Putin and Merkel "emphasised the importance" of Thursday's Geneva talks.
In another statement on Tuesday, the Kremlin described the actions of the Ukrainian army in eastern Ukraine as an "anti-constitutional course to use force against peaceful protest actions".
Kiev's response to the eastern insurgency also prompted Putin to tell UN Secretary General Ban Ki-moon that Moscow "expects clear condemnation from the United Nations and the international community of the anti-constitutional actions" by Ukraine.
Ban in turn "expressed his alarm about the highly volatile situation in eastern Ukraine" and told the Russian leader that everyone involved needed to "work to de-escalate the situation", his office said.
But the White House described Ukraine's military operation as a "measured" response to a lawless insurgency that had put the government in an "untenable" situation.
Washington also said it was coordinating with its European allies to slap more sanctions on Russia over the crisis.
"Our national security team is in active discussions about the next round of sanctions," State Department spokeswoman Jen Psaki told reporters.
She added however that new measures were unlikely before the Geneva talks.
Kiev's untested interim leaders -- who took power in February after four months of pro-European protests ousted Kremlin-backed president Viktor Yanukovych -- have struggled to meet the high-stakes challenge presented by the coordinated series of raids that began in the industrial hubs of Donetsk and Lugansk and have since spread to nearby coal mining towns and villages.
The breakaway move could potentially see the vast nation of 46 million people break up along its historic Russian-Ukrainian cultural divide.
Moscow last month annexed the largely Russified region of Crimea after deploying military forces there and backing a hasty local referendum calling for the Black Sea peninsula to be absorbed into the Russian Federation.
But a forceful military response by Kiev could prompt a devastating counterstrike by Russian troops who are waiting to act on Putin's vow to "protect" Russian-speakers in the neighbouring state.
Ukraine's acting President Oleksandr Turchynov told an agitated session of parliament on Tuesday that the country was facing an eastern enemy rather than domestic discontent.
"They want to set fire not only to the Donetsk region but to the entire south and east -- from Kharkiv to the Odessa region," the acting president said.