U.S. Secretary of State John Kerry took a risky gamble facing derision and much skepticism when he flew to Russia last week for the first time in two years to meet President Vladimir Putin.

But his bet may be beginning to pay off, as two top American diplomats were welcomed in Moscow on Monday for top-level talks for the first time in months on two crises bedeviling global affairs -- Ukraine and Syria.

U.S. Assistant Secretary of State Victoria Nuland called on Monday for concrete progress on a frayed Ukraine peace plan after a flurry of diplomacy with Russia over the crisis rocking the ex-Soviet state. 

Nuland was in Moscow for talks with senior Russian officials that rounded out a week of high-level diplomacy which saw her boss John Kerry jet in to push President Vladimir Putin on implementing the February peace deal. 

Ukraine vowed on Monday to show off two Russian soldiers it claimed to have captured while fighting Moscow-backed forces in the separatist east.

The politically-charged declaration came as a tenuous February truce was broken by more violence that claimed the lives of at least four Ukrainian servicemen.

Ukraine has captured two Russian soldiers in its conflict with separatists in the country's east, Kiev said Sunday.

Military spokesman Andriy Lysenko simply named those captured as "Russian soldiers", while giving no further details. 

Ukrainian President Petro Poroshenko on Friday enacted laws banning Soviet symbols and communist-era propaganda, his office said, after the measures drew strong Russian criticism when they were approved by parliament last month.

The measures come as Ukraine's pro-Western government seeks a complete break with its Soviet past and as its soldiers fight rebels in its east allegedly supported by Russia.

Republican U.S. Senator John McCain has turned down a post as advisor to Ukraine's pro-Western President Petro Poroshenko, saying constitutional constraints prohibited him from accepting.

McCain, who has pressed Washington to send lethal weapons to war-torn Ukraine, rejected the offer to join an advisory council aimed at building global support for Ukraine citing his obligations as a U.S. senator.

A senior aide to Ukrainian President Petro Poroshenko on Thursday accused Facebook of political bias, ahead of a question-and-answer session with co-founder Mark Zuckerberg

"There is pro-Russian influence and pressure on the Facebook office in Russia," said deputy head of the presidential administration Dmytro Shymkiv, who is negotiating with the global social network to solve the problem.

France's President Francois Hollande on Thursday denounced as "unacceptable" ceasefire violations in eastern Ukraine, after speaking briefly with his Ukrainian counterpart Petro Poroshenko in Germany.

"There are ceasefire violations today and that's unacceptable," Hollande told reporters, adding that "there is a risk of fresh unrest and we need to warn them of that".

Ukrainian President Petro Poroshenko has appointed John McCain, a hawkish U.S. senator who has pressed Washington to send lethal weapons to war-torn Ukraine, as his advisor, his administration said.

Ukraine's pro-Western authorities are putting together an advisory council to help the ex-Soviet country conduct reforms and build global support for Ukraine.

The Baltic states will formally ask NATO to deploy several thousand troops as a deterrent to Russia, Lithuania said Thursday, but the alliance gave no assurance that the request would be accepted.

NATO Secretary General Jens Stoltenberg said at meeting of the alliance's foreign ministers in the Turkish city of Antalya that he was aware such a request was being made but emphasized it was too early to assess the demand.