The United States and Russia on Friday launched a round of 11th-hour diplomacy just two days before Crimea votes to secede from Ukraine in a referendum that has sparked the biggest East-West showdown since the Cold War.
US Secretary of State John Kerry met his Russian counterpart Sergei Lavrov in London a day after bloodshed returned to the streets of Ukraine with the stabbing death of a man in clashes between pro-Moscow and pro-Kiev supporters in the eastern city of Donetsk.
"This is a difficult situation we are in," Lavrov told Kerry at the start of their meeting. "Many events have happened and a lot of time has been lost."
Ukraine remained a tinderbox as more than 8,000 Russian troops staged drills near its eastern border while NATO and US reconnaissance craft and fighters patrolled the skies of the ex-Soviet state's EU neighbours to the west.
Kerry has warned Russia that Washington and Europe could announce a "very serious" response as early as Monday if Moscow does not pull back the troops who seized control of Crimea days after the pro-Kremlin regime fell in Kiev last month.
"The first thing that Secretary Kerry will say is 'Will you use your influence to buy time and space for negotiations to take place?," one US official said ahead of the Kerry-Lavrov talks.
Yet Russia has shown little willingness to negotiate and refuses to recognise the legitimacy of the Western-leaning team that has taken power in Kiev -- a move that threatens to shatter President Vladimir Putin's dream of rebuilding a Soviet-type empire.
The diplomatic drama played out before a global audience at the United Nations on Thursday when Ukraine's new prime minister Arseniy Yatsenyuk turned to Moscow's UN representative Vitaly Churkin and asked him directly: "Do the Russians want war?"
Churkin replied that Russia did not. But he also repeated Putin's argument that Yatsenyuk and his allies had conducted the "forceful overthrow" of Moscow-backed president Viktor Yanukovych that created a "government of victors" and not of Ukraine's democratic majority.
Deadly violence meanwhile returned to Ukraine for the first time since nearly 90 people were killed in a week of carnage before the fall of the pro-Kremlin regime as a pro-Kiev protester was stabbed to death in the mostly Russian-speaking city of Donetsk.
The local health service said a 22-year-old man was killed and 16 people were wounded in unrest that erupted when pro-Kiev demonstrators were attacked by pro-Moscow protesters.
Footage on Ukrainian television showed mass fistfights breaking out and clubs being wielded as a much smaller presence of helmeted riot police stood in the middle of the melee and seemed incapable of separating the crowds.
Ukraine's acting President Oleksandr Turchynov blamed the death on separatists "sent in" from Russia.
"These people and the Kremlin do not care about the lives of those they claim to be protecting," Turchynov said in a statement.
- 'Very serious' response -
Sunday's vote gives residents of Crimea -- a Russian-speaking region that has housed tsarist and Kremlin navies since the 18th century -- only two choices: joining Russia or "the significant strengthening of their autonomy within Ukraine".
The Black Sea peninsula's self-declared pro-Kremlin leader has already predicted an easy victory and the region is largely expected to vote in favour of joining Russia despite discontent from the Muslim Tatar minority that makes up 12 percent of Crimea's total population of two million.
Tatar community leader Mustafa Dzhemilev told AFP on Thursday that NATO should intervene in Crimea to avert a "massacre" of his people by the Russians.
But Washington and its European allies are far more likely to stiffen sanctions against top Russians should the Kremlin fail to scale down its military involvement in Crimea and open direct dialogue with Kiev.
"If there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps on Monday in Europe and here with respect to the options that are available to us," Kerry told lawmakers in Washington.
The prospect of imminent sanctions saw the Moscow stock exchange lose four percent of its value in afternoon trading. The ruble was also changing hands near historic lows despite the central bank's injection of $1.5 billion (one billion euros) into the market in an effort to prop up the flagging currency.
- Military buildup -
The European Union will debate travel bans and asset freezes on Monday against Russian officials held responsible for threatening Ukraine's territorial integrity.
The White House has been moving toward punitive measures faster than its European allies -- whose financial and energy sectors are tightly intertwined with Russia's -- and has already approved visa restrictions and financial penalties on Moscow officials.
But US President Barack Obama told Yatsenyuk after talks in the Oval Office that Washington was willing to move much further still if Putin failed to soften his stance immediately.
Ukraine on Thursday created a new National Guard of 60,000 volunteers to supplement a conventional army of 130,000 soldiers that is dwarfed by a 845,000-strong Russian force backed by nuclear arms.
Russia's tanks and artillery units are conducting exercises across three regions neighbouring Ukraine while 4,000 paratroopers are performing drills in the central region of Rostov.
But some analysts said it will be difficult for Putin to emerge the victor in his latest standoff with the West.
"In any scenario, the ultimate loser... will very likely be Putin," said Holger Schmieding of Germany's Berenberg Bank.
"While he can get some short-term popularity boost now by annexing Crimea, his fragile petro-state cannot afford the costs of sanctions, let alone the costs of war."