Ukraine’s Western-backed leader on Tuesday accused Russia of trying to inflame the country’s southeast but said he would proceed cautiously against pro-Kremlin militias consolidating control in the volatile region.

Oleksandr Turchynov’s impassioned charges against Ukraine’s historic master came only hours after a “frank and direct” exchange on the crisis between US President Barack Obama and his Russian counterpart Vladimir Putin.

But the heated phone conversation appeared to break no new ground as the Kremlin chief continued to reject any links to the Russian-speaking gunmen who have occupied town halls and police stations in nearly 10 cities across Ukraine’s struggling eastern rust belt since the start of the month.

European foreign ministers meanwhile held back on unleashing punishing economic sanctions against Russia in hopes that EU-US mediated talks Thursday in Geneva between Moscow and Kiev could help deescalate the most explosive East-West standoff since the Cold War.

But the pressure that the ex-Soviet state’s interim leaders are feeling from Moscow is now also starting to reverberate from their supporters in Kiev who had toppled a detested pro-Kremlin regime in February after months of protests that sought to link up Ukraine firmly to the West.

Displeasure at Ukrainian forces’ thus-far helpless efforts to reassert control and anxiety over their country’s possible breakup saw several hundred nationalists set fire to tyres outside the parliament building on Monday evening demanding the interior minister’s resignation.

Turchynov appeared to address that discontent on Tuesday as he stressed that his “full-scale anti-terrorist operation” that aims to dislodge the pro-Russian gunmen from their increasingly entrenched positions must proceed “gradually, responsibly and in a measured way”.

NATO chief Anders Fogh Rasmussen urged Russia Tuesday to de-escalate tensions over Ukraine and withdraw troops massed on the border with its Soviet-era satellite.

“I am deeply concerned by the latest developments in Ukraine,” Rasmussen said, citing the seizure of government buildings in the east of the country by pro-Moscow militias.

“I call on Russia to de-escalate the crisis, to pull back its troops … to stop destabilizing the situation and make clear it does not support the violent actions of pro-Russian separatists.”

“Russia should stop being part of the problem and start being part of the solution,” he said.

Rasmussen was to brief European Union defense ministers meeting in Luxembourg on NATO measures to bolster alliance member states in eastern Europe where the Ukraine crisis has sparked fears Russia is seeking to re-establish its former sphere of influence.

The NATO chief stressed “we are not discussing military options.”

“We believe the right way forward is to find a political and diplomatic solution,” he added.

Turchynov told an agitated session of parliament that saw some of his old protest supporters question his leadership that Ukraine was facing an eastern enemy rather than domestic discontent.

“Russia had and continues to have brutal plans,” said Turchynov.

“They want to set fire not only to the Donetsk region but to the entire south and east — from Kharkiv to the Odessa region.”

He added that Donbass — the informal name of the Donetsk region that is now the hotbed of Ukraine’s latest wave of unrest — had a vast majority that was happy to be rid of the old leaders and become part of a broader Europe.

“The Donbass is in colossal danger,” said Turchynov.

“Besides the Russian special special forces, besides terrorists, Donbass also has hundreds of people who have been deceived by Russian propaganda,” he argued.

“And besides them are hundreds of thousands of completely innocent Ukrainians. That is why an anti-terrorist operation must be carried out responsibly.”