Russia said its military is massed on Ukraine's doorstep and warned against further US sanctions as a deal struck with Washington appeared to stall because of intransigence by Moscow-backed rebels in the former Soviet republic.
A threat by US President Barack Obama that more sanctions would befall Moscow if the agreement, reached Thursday with Ukraine and the EU, failed was "absolutely unacceptable," Kremlin spokesman Dmitry Peskov told Russian television.
"Our Western colleagues are trying to push responsibility towards our side. But it must be underlined: it is a collective responsibility," he said.
He pointedly declared that "there are troops close to the Ukrainian border". "Some are based there, others have been sent as reinforcements due to the situation in Ukraine," he added.
The comments appeared to be a warning shot to Washington that the situation could quickly degrade if Moscow were punished for a failed implementation of the accord.
The White House said it was watching Moscow to see if it is holding up its end of the deal.
"We expect and we will be watching whether Russia does or does not uphold its responsibility to use its very considerable influence to restrain and withdraw those irregular militia from the buildings and spaces that they've occupied," said National Security Advisory Susan Rice.
In a phone call on Friday with Russian Foreign Minister Sergei Lavrov, US Secretary of State John Kerry said the following days would be "pivotal", a senior State Department official said.
The top US diplomat stressed that "all illegal armed groups must be disarmed and all illegally seized buildings must be returned to legitimate owners," according to the official.
But on the ground in Ukraine's restive east, there were signs the pact was coming unstuck hours after the four parties signed off on it.
Pro-Russian rebels were refusing to cede control of a string of towns they seized over the past two weeks.
But, says Moscow, neither are pro-Western protesters in Kiev budging from the iconic Maidan square they continue to occupy even after ousting former president Viktor Yanukovych two months ago.
"One cannot treat Russia like it is a shameful student, and brandish a piece of paper on which we put crosses next to the duties it has accomplished," said Peskov.
If the hard-won agreement collapses -- as Western and Ukrainian leaders fear it might -- the United States has said it will quickly ramp up sanctions on Russia, which it holds responsible for supporting the separatists and stoking the crisis.
Defiance in the east
In the main eastern city of Donetsk, where rebel gunmen wearing ski-masks still occupied a barricaded government building, there was defiance, with the Russian national anthem blaring out through speakers.
Denis Pushilin, a prominent member of the self-declared Donetsk Republic, said he agreed that the buildings should be vacated, but only if the leaders in Kiev must also leave the buildings "that they are occupying illegally since their coup d'etat" in February.
In nearby Slavyansk, insurgents remained holed up inside a seized police station.
Unyielding pro-Russian militants also manned barricades of tyres and sandbags in another town of Kramatorsk while Ukrainian military helicopters continued to land at a nearby aerodrome that remains under Kiev's control.
In a concession to the Russian-speaking militants, Ukrainian Prime Minister Arseniy Yatsenyuk pledged safeguards for the Russian language and a broad decentralisation of power.
Ukraine's Foreign Minister Andriy Deshchytsya also suggested the authorities would hold off temporarily from any further military action on the ground to give the agreement time to take effect.
"If this does not begin in the next few days then I think after Easter there will be more concrete actions," Deshchytsya told journalists. Easter in Orthodox Ukraine ends on Monday.
A sceptical Obama, likewise, has spoken of wanting to see progress within days, otherwise further sanctions, on top of those already targeting Vladimir Putin's inner circle, would be implemented. US Vice President Joe Biden is due in Kiev on Tuesday.
The US president also on Friday made clear his "disgust" at reports of anti-Jewish leaflets handed out in Donetsk.
The pamphlets telling Jews to register or be expelled sparked global outrage and fears of a Nazi-style pogrom.
The Ukrainian government and many Western states believe the occupations in the east took place with the active support of elite Russian military units, allegations Moscow denies.
To back their claims, Ukraine said it was holding 10 Russian "spies" it believes were sent to stir up unrest. It has also, according to the Russian airline Aeroflot, banned the entry of Russian males aged 16 to 60.
In a rare success, the Ukrainian army said it had recovered two of six armoured vehicles captured by separatists during a disastrous military operation earlier in the week.
No high hopes
On Thursday, Putin warned that Ukraine was plunging into the "abyss" -- and he hoped he would not have to resort to his "right" to send in the tens of thousands of troops deployed on Ukraine's border.
He blames the turmoil on Kiev's interim leaders, whose authority he does not recognise.
Some analysts say the agreement could be a face-saving solution for Russia.
"I can see why they (Russians) did this as they felt that the sanctions were quite close to being imposed, so they had to take a step back," Kiev-based political scientist Andreas Umland said.
"But I'm sceptical... I don't think it's all over."
Anatoliy Gritsenko, a former Ukrainian defence minister and a candidate for next month's presidential election, wrote on his website: "Will the Geneva agreement calm the situation in Ukraine? My response is no."