British Prime Minister David Cameron arrived in Moscow 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 Monday with Russian President Dmitry Medvedev, and is expected to raise the case during their talks. Mr. Cameron is also scheduled to meet with Russian Prime Minister Vladimir Putin. The meeting with Mr. Putin will be the first by a British official since 2007.

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.