Russia says its military is massed on Ukraine’s doorstep and it has 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 on Thursday with Ukraine and the EU, failed was “absolutely unacceptable,” Kremlin spokesman Dmitry Peskov said.

“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 deteriorate if Moscow were punished for a failed implementation of the accord.

The White House reiterated 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.

Her colleague at the State Department, spokeswoman Jen Psaki added: “If they don’t take steps, there will be consequences.”

The agreement included a call on “all illegal armed groups” to disarm and leave seized state buildings and squares.

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 neither are pro-Western protesters in Kiev budging from the iconic Maidan square, says Moscow.

The protesters continue the occupation 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. It holds Russia responsible for supporting the separatists and stoking the crisis.

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.

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 president expressed his disgust quite bluntly,” said Susan Rice.

“I think we all found word of those pamphlets to be utterly sickening and they have no place in the 21st century.”

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.