The permanent council of the world's largest security body will meet in Vienna on Thursday in a bid to push Russia, the US and its European allies towards talks to defuse the mounting crisis on Ukraine's border.
Ahead of Thursday's meeting of the Organisation for Security and Co-operation in Europe (OSCE) in Vienna, the West has accused Russia of massing tanks, artillery and about 100,000 soldiers on Ukraine's war-torn eastern border in recent weeks, in what Nato says is preparation for an invasion.
But Russia insists the military deployment is a response to what it sees as the growing presence of Nato in its sphere of influence, where it fiercely opposes the expansion of the Atlantic alliance.
Russia and the US have already underlined their "fundamental" differences on European security during tense talks in Geneva and Brussels this week.
And the US has admitted no breakthrough is expected at Thursday's meeting.
"I don't think there will be any concrete results this week. Our main goal is, in principle, to establish a dialogue," said Michael Carpenter, US ambassador to the OSCE.
"Yes, our positions are polar, but this does not mean that there are no elements and areas on which we cannot agree," he told independent Russian TV channel Dozhd.
The challenge, he said, will be to "determine in what forms it is generally possible to deepen the dialogue on this issue in the next few months or even a year".
Situation 'extremely worrying'
The OSCE, a multilateral forum for East-West discussions during the Cold War, is uniquely suited to walk this complex diplomatic tightrope.
Thursday will be its first meeting of the year, attended by all 57 member states.
After the morning presentation of the year's priorities by the new Polish presidency, OSCE Secretary General Helga Schmid will hold a press conference at 12.30pm.
Talks are complicated by the unclear situation on the ground in rebel-held eastern Ukraine, where the OSCE has since 2014 been charged with ensuring peace accords are respected.
That has failed to end fighting in the region, however, with conditions degrading for OSCE observers in areas controlled by pro-Russian separatists, a situation the US ambassador called "extremely worrying".
"The monitoring missions have not yet recorded anything anomalous", said Mr Carpenter, while admitting that on the border "we cannot possibly know what is actually happening".
