Ukraine is braced for a highly-charged breakaway vote in Crimea which is likely to see the peninsula annexed by Russia despite the threat of stiff sanctions, and push East-West tensions to breaking point.
An eleventh-hour diplomatic push by the United States to stop Sunday's referendum from going ahead failed on Friday, with Moscow refusing to make any decisions until after Crimea votes on a spilt from Ukraine on Sunday.
The majority Russian-speaking Black Sea peninsula has been given only two choices in a referendum hastily called by pro-Moscow authorities after a street revolt saw pro-EU protesters seize power in Kiev.
Crimeans can either vote to join Russia or for "the significant strengthening of their autonomy within Ukraine".
World leaders are likely to step up pressure in the final hours before Sunday's vote, expected to swing easily in favour of Russia despite discontent from the Muslim Tatar minority that makes up 12 percent of Crimea's total population of two million.
The UN Security Council will vote Saturday on a resolution on Ukraine denouncing the referendum.
"The only objective of a resolution would be to get the Chinese to abstain and isolate Russia further," a council diplomat said.
UN Secretary General Ban Ki-moon called Russian President Vladimir Putin Friday and afterwards said he still thought a negotiated solution was possible, although it was "clear we are at a crossroads".
However with the result of the vote seen as a foregone conclusion, the international community is playing wait-and-see as to whether Moscow will actually take the extra step of annexing the region of two million people.
US Secretary of State John Kerry warned Russia against accepting the results of the vote, saying this would amount to a "backdoor ratification" -- a move that would trigger sanctions and escalate the biggest East-West showdown since the 1989 fall of the Berlin Wall.
"If the referendum takes place, there will be some sanctions; there will be some response, put it that way," Kerry told reporters after last-ditch, six-hour talks with Russian Foreign Minister Sergei Lavrov.
Lavrov hinted that Moscow was now resolved to bring Crimea under its eventual control.
"Everyone understands – and I say this with all responsibility – what Crimea means to Russia, and that it means immeasurably more than the Comoros (archipelago) for France or the Falklands for Britain."
The top diplomats failed to see eye to eye on a crisis which has seen Russian troops invade Crimea while Moscow stages military drills near Ukraine's eastern borders, warning it reserves the right to protect compatriots in the whole of the country.
Clashes in the east Ukrainian city of Donetsk on Thursday which left a pro-Kiev protester dead and several injured, saw the Kremlin charge that "Kiev authorities do not have the situation under control."
The former Soviet republic of some 46 million people remains a tinderbox, having undergone rapidfire changes since president Victor Yanukovych was ousted in February by a protest movement that raged for months after he rejected closer ties with the EU.
Russia still refuses to recognise the legitimacy of the Western-leaning team that has taken power in Kiev -- a move that threatens to shatter Putin's dream of rebuilding an empire dominated by Moscow.
More than 8,000 Russian troops are staging drills near Ukraine's eastern border while NATO and US reconnaissance aircraft and fighters patrol the skies of its EU neighbours to the west.
A Russian state arms and technology group, Rostec, said Friday it had intercepted a US surveillance drone above Crimea flying at about 4,000 metres (12,000 feet).
British Foreign Secretary William Hague said the failure of the talks was "deeply disappointing" and called for "tougher restrictive measures".
The European Union will debate travel bans and asset freezes on Monday against some 30 Russian and Ukrainian officials held responsible for threatening Ukraine's territorial integrity.
Asked when the United States would respond to the referendum, White House spokesman Jay Carney said: "I think, without putting too fine a point on it, I'd say quickly."
Kerry insisted again that Washington did not want to impose sanctions on Moscow, but said even just the threat was causing Moscow stocks to tumble.
They plunged to a four-year low Friday as jittery investors dumped their holdings ahead of the referendum, while the ruble again fell against major currencies.
US stocks also took a hit while oil prices rose.