US Secretary of State Condoleezza Rice on Thursday called on Russia to honour Britain's request to extradite the chief suspect over the murder of former agent Alexander Litvinenko.
Speaking shortly before Russian announced the tit-for-tat expulsion of four British diplomats, she urged "full cooperation" from Moscow over London's extradition request for Andrei Lugovoi.
"Russia should honour the extradition request and Russia should cooperate fully," she told Britain's Sky News television, in the first high-level US reaction to the rapidly-worsening standoff between Britain and Russia.
"A terrible crime was committed on British soil and Britain has to find the perpetrators and bring them to justice... It is not in anybody's interests that you can have a crime committed of this kind and nothing be done about it.
"I would hope and we are encouraging that there should be full cooperation from Russia -- and indeed extradition," she added, in an interview conducted in Lisbon.
The Russian constitution forbids Russia from extraditing its own citizens, but Britain insists that Moscow could extradite Lugovoi under the terms of an international convention it has signed.
Rice said there were some "very, very big troubles" with Russia, but stressed that the country was a key strategic partner and had come a long way since the end of the Soviet Union.
"Russians live in greater freedom, probably, personal freedom than they ever have," she said.
"We have wide areas of cooperation with Russia; on terrorism, on nuclear non-proliferation. I think we will find answers to the threats that we face.
She said the US was "working very well on North Korea. We feel we work well together on Iran and certainly we have put forward some proposals, as have the Russians, on missile defence preparations.
"So it's a very different world than the Soviet Union. But there are also complexities.
"We have all been disappointed that there has not been more rapid institutionalisation and progress of democracy in Russia.
And she said: "We all are concerned that Russia not use its energy resources somehow as a political tool but rather engage in an open international economy in which can oil and gas can fuel economic growth for everyone.
"There's some very, very big troubles, but this is not the Soviet Union."
