DONETSK (AFP) – US Secretary of State John Kerry will on Friday hold crunch talks with Russia in a bid to defuse tensions ahead of a breakaway vote in Crimea, as street battles turned deadly in eastern Ukraine.

Kerry has warned Moscow of a serious backlash over Sunday’s referendum, which is widely rejected as illegitimate and could see the Black Sea peninsula slip from Ukrainian control and into Russia’s grasp.

The top US diplomat will meet his Russian counterpart Sergei Lavrov in London for tough talks after several recent diplomatic clashes in what has mushroomed into the most bitter East-West showdown since the end of the Cold War.

Both Ukraine and Moscow insisted in front of the UN Security Council on Thursday that they did not want war, even as tensions over the advance of Russian forces in Crimea claimed their first victim when a pro-Kiev protester was stabbed and killed in the eastern city of Donetsk.

The neighbours have been locked in an escalating stand-off since February 22, when a wave of street revolts ended up ousting Ukraine’s pro-Kremlin president Viktor Yanukovych.

Russian President Vladimir Putin refuses to recognize Ukraine’s new Prime Minister Arseniy Yatsenyuk’s new pro-Western administration, and tensions are building between rival camps inside Ukraine.

Following Yanukovych’s ouster, Russian troops quickly seized control in Crimea — a small peninsula dangling between the two nations which has often changed hands in its turbulent past.

Pro-Moscow authorities then organised a breakneck referendum in the flashpoint region which is home to a mainly ethnic Russian population and Russia’s Black Sea fleet.

Sunday’s vote gives Crimean residents only two choices: joining Russia or “signficant strengthening of their autonomy within Ukraine”.

The vote is seen as likely to bring the simmering crisis to a head.

Addressing an emergency session of the Security Council, Yatsenyuk urged Russia to negotiate an end to the stand-off.

“We want to have talks. We don’t want to have any kind of military aggression,” he insisted, turning to directly address Russia’s UN ambassador Vitaly Churkin.

Churkin ridiculed the idea that there had been an “idyllic situation” before the crisis, but said: “Russia does not want war and nor do the Russians, and I’m convinced that Ukrainians don’t want this either.”

Sanctions expected

Backed by European powers and Washington, Yatsenyuk has a strong diplomatic hand.

But the facts on the ground favour Putin.

His army is far larger than Ukraine’s and there appears to be little appetite to challenge him militarily.

Instead, Washington and the European Union are preparing what Kerry called “a very serious series of steps” against Russia.

A senior US official insisted Washington is “opening the door” to negotiations with Russia. But asked if it was a forgone conclusion that US and EU sanctions would be imposed on Monday, she said: “Yes.”

The official told reporters travelling with Kerry that he would seek “commitment to stop putting new facts on the ground and a commitment to engage seriously on ways to de-escalate.”

The official said Kerry will also question massive military drills Russia is carrying out on its neighbour’s doorstep.

“This is the second time in a month that Russia has chosen to mass large amounts of force at short notice around the eastern borders of Ukraine,” the senior US official said.

“It certainly creates an environment of intimidation. It certainly is destabilising and that will be one question that is asked tomorrow, what is meant by it,” she added.

Washington’s stern warnings came after Ukraine’s parliament voted to set up a huge volunteer force.

Meanwhile, in eastern Ukraine, rival protests between pro- and anti-Russian factions have turned bloody, threatening to draw the opposing security forces into conflict in the streets.

A 22-year-old Ukrainian was killed when people at a pro-Kiev rally in Donetsk were attacked by a pro-Moscow group. It was the first confirmed death since Crimea was seized.

“According to preliminary conclusions by doctors, he has been stabbed,” the local branch of Ukraine’s health ministry told AFP, as regional authorities reported another 16 wounded in the clashes.

Thirteen of these were in trauma or in surgery with serious injuries, one was hospitalized and two were treated at the scene, said Ilya Suzdalev, a spokesman for the regional authorities.