British Prime Minister David Cameron is to arrive in Moscow later Sunday on the first official visit by a British leader to Russia in six years.

Mr. Cameron is expected to encourage closer trade ties with Russia. Relations between the two countries soured in 2006 after a Russian ex-security agent, Alexander Litvinenko, died of poisoning in London.

So far, Russia has refused British requests to extradite Andrei Lugovoy, a Russian suspected of being responsible for Litvinenko's death.

Mr. Cameron is to meet Russian President Dmitry Medvedev, and is expected to raise the case during their talks.

But trade appears to be at the top of Mr. Cameron's agenda. Traveling with the British prime minister is BP senior executive Robert Dudley.

On Sunday, Russia's embassy in London said its website had crashed, in what appeared to be a hacking attack. The embassy said it had set up a “mirror” site.