Mr Kerry is 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'BBC diplomatic correspondent James Robbins says that London is now the stage for a revival of Cold War tensions.The American and Russian foreign ministers will meet at the US ambassador's residence in central London.Our correspondent says they will have very different accounts of events in Ukraine.Mr Kerry will try to persuade Russia that it risks paying a heavy price in political and economic damage from American and European measures which could be triggered by Sunday's referendum."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," Mr Kerry said prior to leaving for London.Our correspondent says that while Mr Kerry seems to think the referendum itself may be all but unstoppable, he insists that it is what Russia does after that vote which counts - and Ukraine's territorial integrity must not be permanently violated.The talks are the last opportunity for face-to-face dialogue at such a senior level before the likely vote in Crimea, our correspondent says, which could determine whether what happens next edges Ukraine away from, or deeper into, a dangerous crisis.Mr Kerry told lawmakers before his departure to London that the US was 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 secretary of state said that he had spoken again by telephone with Mr Lavrov on Thursday ahead of their Friday meeting, and that he and his Russian counterpart had been in almost daily contact over the past two weeks.Mr Kerry has hinted at a possible compromise to the crisis by which the Ukrainian parliament would allow 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."The constitution of Ukraine requires that any effort by any entity within Ukraine to secede be done through the constitutional process," Mr Kerry said.He said that at the moment Russia did not "have the assets... necessary to be able to march in and take over Ukraine", although he conceded that could change in future.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 weaponsA young man was stabbed to death and more than a dozen people were in hospital after pro-Russian and pro-European demonstrators clashed in the eastern Ukrainian city of DonetskIn what it called a "positive development", the Organisation for Security and Co-operation in Europe (OSCE) on Thursday said that Russia had now given its support to a possible long-term monitoring mission in Ukraine(BBC)ANN.Az