Ukraine's interim president on Monday made a dramatic about-face aimed at defusing tensions in the separatist east by backing a national referendum on turning the ex-Soviet republic into a federation with broader regional rights.

European powers meanwhile sought to raise the pressure on Russia -- which it blames for fomenting the tensions -- with Britain calling for "further sanctions" on Moscow ahead of a meeting of EU foreign ministers.

And Germany signalled its own resolve to take a tougher stance against a partner from which it imports 40 percent of its gas by noting there were "many signs" that armed groups operating in eastern Ukraine were "receiving support from Russia".

Interim president Oleksandr Turchynov's u-turn came only hours after pro-Kremlin militants who reject the authority of the new Western-backed leaders ignored an ultimatum to end their occupation of strategic buildings or face a "full-scale anti-terrorist operation" involving both internal security forces and army troops.

The coordinated raids and dual threat posed by Russia's deployment of 40,000 troops on Ukraine's border and warning of a possible gas cutoff have left Kiev's untested leaders desperately seeking Western help in averting a further dismemberment of their crisis-hit state.

EU foreign ministers -- their capitals bracing for what might be the third halt in Russian gas supplies since 2006 -- gathered in Luxembourg to discuss whether to pursue a third and most punishing-yet round of economic sanctions against Moscow.

But Washington has also advised Kiev to devolve powers in order to remove any argument Moscow might make about discrimination against Russian speakers -- a charge that has fed fears of a further invasion following the Kremlin's annexation of Crimea last month.

Turchynov had long and furiously resisted Russia's idea of turning Ukraine into a federation. But he said on Monday that he was ready to put it up for a national vote to prove that most shared his view.

"In recent days, there has been a lot of talk about a national referendum," Turchynov told leading lawmakers in nationally televised remarks.

"We are not against holding a national referendum that -- if parliament adopts the corresponding decision -- could be held together with the presidential elections," Turchynov said in reference to the May 25 vote.

"I am certain that a majority of Ukrainians will support an indivisible, independent, democratic and united Ukraine," he added. "This is my conviction, and I think that all those present share my view."

Pro-Kremlin protesters in rundown eastern rust belt regions such as Donetsk and Kharkiv are seeking local referendums on either broader rights or an option to join the Russian Federation.

Turchynov's announcement stops well short of meeting those demands and it remains unclear how the gunmen -- or Russia -- intend to respond.

The outcome of a national vote on federalisation is uncertain because most in Kiev and the Ukrainian-speaking west share more nationalist ideals and support a strongly unified state.

'Naked aggression'

The pro-Kremlin gunmen's latest raids were especially unsettling for Kiev and Western leaders because of their remarkable similarity to events leading up to Russia's annexation of Crimea.

The balaclava-clad gunmen were armed with special-issue assault rifles and scopes most often used by nations' crack security troops.

Many wore unmarked camouflage uniforms similar to those seen on the highly trained units that seized the Black Sea peninsula in early March. They also moved with military precision and cohesion.

Turchynov had warned on Sunday evening that he would launch a "full-scale" assault against the militants if they failed to give up by Monday morning.

AFP reporters across the Donetsk region saw no signs of a Ukrainian offensive.

The streets of Slavyansk -- a rundown coal mining town of 100,000 that has been under the militants' effective control since Saturday -- were deserted and silent except for a crowd of 1,000 that had gathered near the seized state buildings to show their support for the insurgents.

Protesters had set up road checks along the main highway leading into the city while many inside Slavyansk itself spoke in favour of joining Kremlin rule.

"We would have preferred autonomy within Ukraine," said a 46-year-old teacher named Oleksandr. "But under current circumstance, we are seeking unification with Russia."

But Donetsk Governor Sergiy Taruta told local residents in an official statement that the army operation had already begun.

"They are terrorists and we will not allow them to lord over our land," the Donetsk governor said.

Kiev 'stooges'

Western and Kiev charges of its involvement in the latest wave of east Ukrainian unrest prompted Moscow to convene an emergency session of the UN Security Council whose charged atmosphere echoed diplomatic battles waged at the height of the Cold War.

Moscow's UN ambassador Vitaly Churkin said in a prepared statement that "the international community must demand the stooges of Maidan stop the war against their own people."

Maidan refers to the barricade-scarred protest square in Kiev that witnessed dozens of deaths during months of protests that toppled an unpopular Russian-backed leader in February.

But US ambassador Samantha Power told the 15-member council sternly that Ukraine faced "the saddest kind of instability. It is completely man made. It was written and choreographed in and by Russia."