The Russian foreign minister, Sergei Lavrov, appears to have offered a deal to resolve the crisis in eastern Ukraine, suggesting that if the country's government clears out the nationalist protest camp in Kiev, then pro-Moscow separatists will lay down their arms.

Western officials greeted the proposal with scepticism, noting that such confidence-building measures were at the heart of an international agreement reached last week, but which failed to end the separatists' occupation of public buildings in eastern Ukraine. They said the protest camp in Independence Square in Kiev, erected in February during the uprising that toppled the Russian-backed president, Viktor Yanukovych, was already being dismantled.

The Organisation for Security and Co-operation in Europe, which is monitoring the situation in Ukraine, reported that its team in the capital "observed the ongoing clearing of barricades in the Maidan square".

"The situation in the capital city was calm," the report added.

One western official raised the possibility that Lavrov might be seeking to use the dismantling of the camp as a face-saving way out of the crisis, but cautioned that there were few other signs of compromise from Moscow.

Lavrov's comments came as the Ukrainian government launched further military operations against some of the pro-Russian separatists who have seized government buildings across eastern Ukraine, having killed up to five rebels on Thursday.

He said the pro-Russian militias in the east "will be ready" to lay down arms and vacate the buildings "only if Kiev authorities get down to implementing the Geneva accords, clear out that shameful Maidan [Independence Square] and liberate the buildings that have been illegally seized".

Lavrov spoke after Barack Obama told reporters in Seoul he would call European leaders during the course of the day to discuss action since the failure of the Geneva agreement, a possible prelude to new US and EU sanctions against Russia.

The crisis is already showing signs of seriously damaging the Russian economy. The Standard & Poor's credit agency cut the country's credit rating on Friday for the first time in more than five years, following concerns about capital flight and risks to investments.

In unusually blunt language, the US secretary of state, John Kerry, said on Thursday that unless Moscow took immediate steps to de-escalate the situation in Ukraine, Washington would have no choice but to impose additional sanctions. He said it would be a grave and "expensive mistake".

Kerry's comments came amid an increasingly bitter war of words over who is to blame for the crisis, which is becoming steadily more violent.

The Ukrainian prime minister, Arseniy Yatsenyuk, made the strongest comments yet on Friday when he accused Russia of trying "to start world war three" and acting like a gangster.

The US says Russia is fomenting unrest and separatist sentiment in eastern Ukraine following its annexation of the strategic Crimean peninsula. Russia accuses the US of encouraging a pro-western government in Kiev to adopt anti-Russian policies.

The US accused Russia of reneging on the Ukrainian peace deal on Thursday, as both sides insisted on contradictory explanations of clashes overnight on Wednesday with pro-Russia militants.

In its most explicit comments yet on the apparent collapse of last week's agreement, the US state department claimed it had seen no sign of Russia abiding by its commitments.

"Since Geneva, Russia has failed not only to provide public support for the de-escalation of tensions but has actively stoked tensions in eastern Ukraine by engaging in inflammatory rhetoric," the state department spokeswoman, Jen Psaki, told reporters.

"The secretary of state has spoken with foreign minister Lavrov six times since Geneva and he has never once taken responsibility for the implementation of Russia's Geneva commitments and he has gone so far as to say the Geneva agreement demands no action from Russia."

Officials in Washington angrily rejected Moscow's characterisation of clashes with Ukrainian soldiers that raised tensions between the two cold war foes to dangerously high levels.