US Secretary of State John Kerry (centre) met British Prime Minister David Cameron and Foreign Secretary William Hague ahead of talks with Sergei Lavrov. Photo: BBC

Mr Kerry was expected to warn Mr Lavrov that the referendum and Russia's military intervention in Crimea could trigger concerted US and EU sanctions.

He has warned of "very serious steps" if Russia annexes the region.

Russia insisted at the UN on Thursday it did "not want war" with Ukraine.

During an emergency meeting of the Security Council, Moscow's ambassador to the UN Vitaly Churkin defended the right of Crimea, which is predominantly ethnic Russian, to decide whether or not to join the Russian Federation.

Russia's military intervention followed the fall of Ukraine's pro-Moscow President Viktor Yanukovych on 22 February.

'Serious steps'

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• Maps chart Crimea's troubled past

Mr Kerry had talks with UK Prime Minister David Cameron before his meeting with the Russian foreign minister at the US ambassador's residence in central London.

"We want to see Ukrainians and the Russians talking to each other and if they don't then there are going to have to be consequences," Mr Cameron told reporters during the meeting in Downing Street.

Before his arrival in London, Mr Kerry urged movement on the Crimea referendum issue.

"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 [in Washington] with respect to the options that are available to us."

In a separate development on Friday, Russia said it reserved the right to protect "the lives of compatriots and fellow citizens in Ukraine" in a foreign ministry statement, referring to deadly clashes in eastern Ukraine on Thursday.

Russia also called on the Organisation for Security and Co-operation in Europe (OSCE) to send observers to monitor the Crimea vote.

The Russian foreign ministry said the logistics of the mission should be in agreement with "the leadership of Ukraine's regions where (the mission) is expected to be deployed".

Sanctions

Mr Kerry has also said the US is not eager to impose further sanctions on Russia.

"Our choice is not to be put in the position of having to do that. Our choice is to have a respect for the sovereignty and independence and territorial integrity of Ukraine," he said.

The talks between the two foreign ministers are taking place as the eastern city of Donetsk became embroiled in the worst violence since the fall of President Viktor Yanukovych

There is also tension on Ukraine's border with Crimea as the referendum looms

Mr Kerry has said that the outcome of Sunday's referendum in Crimea is not in much doubt, given the peninsula's historic ties to Russia and the fact that it is home to Moscow's Black Sea Fleet

The Pentagon has tried to reassure its Nato allies and partners bordering Ukraine by increasing air patrols in those areas

The secretary of state has hinted at a possible compromise.

This would see the Ukrainian parliament allowing Crimea to hold a referendum on self-determination - similar to Scotland's forthcoming vote in September on whether to bring an end to its 300-year old union with England.

 

Russia's ambassador to the United Nations Vitaly Churkin (R) responds to Ukraine's PM Arseniy Yatsenyuk in New York

"The constitution of Ukraine requires that any effort by any entity within Ukraine to secede be done through the constitutional process," Mr Kerry said.

But correspondents say that the signs are not good for Friday's talks, as both men have clashed in recent weeks and failed to agree on a number of US proposals.

Russia has refused to recognise the interim leadership that took over in Kiev with Mr Yanukovych's departure or participate in a contact group aimed at bringing the two countries together for talks.

'Legal vacuum'

In his appearance before the UN on Thursday, Mr Churkin said that it was Kiev that was "splitting its country into two parts", not Moscow.

The referendum in Crimea, he said, had come about because of a "legal vacuum" in the country, and questioned why Crimeans should not be "afforded the opportunity" to decide on their future.

Ukraine's interim Prime Minister Arseniy Yatsenyuk told the UN Security Council that his country was a victim of Russian aggression, producing a copy of the UN Charter to make his point that Moscow was violating it and several other international treaties.

The BBC's Nick Bryant, in New York, says the US is circulating a resolution stating that Sunday's referendum in Crimea has no validity.

He says it knows that Russia will exercise its veto, but hopes that China will not block it, thus underscoring Russia's diplomatic isolation.

In other developments, Russia launched new military exercises near its border with Ukraine on Thursday involving more than 8,000 troops and large artillery units such as rocket launchers and anti-tank weapons.

Meanwhile, Russian media on Friday reported that Russian fighter jets and helicopters had started training flights over the Mediterranean Sea.

A spokesman for the Russian navy told Interfax news agency - quoted by Reuters - that the Russian aircraft carrier, Admiral Kuznetsov, was involved in the training exercises.

It comes a day after some of the worst violence in Ukraine since the fall of Viktor Yanukovych, with one man killed and a dozen others injured in rival rallies in the eastern Ukrainian city of Donetsk.