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 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.