Vladimir Putin and Prime Minister David Cameron will discuss Syria on Thursday when the Russian president makes his first visit to Britain in seven years, before watching the judo at the Olympics.

The two leaders will hold talks at Downing Street on the Syrian crisis as well as their rocky bilateral relationship before heading together to watch the judo, a sport in which Putin is a black belt.

Relations between Britain and Russia went into deep freeze after Kremlin critic Alexander Litvinenko was killed by radiation poisoning in London in 2006, but Cameron visited Moscow last year in a bid to repair ties.

However, Russia's staunch opposition to UN action against Bashar al-Assad's regime has sparked renewed tension.

Russia and China have jointly blocked three UN Security Council resolutions that would have sanctioned Assad's government for its crackdown on the opposition, and have refused to join international calls for his departure.

Britain has strongly criticised Russia's position on Syria, where fierce fighting between government forces and rebels is raging in the commercial capital Aleppo.

Cameron is among the Western leaders calling for Assad to step down and Britain has warned of an impending humanitarian disaster in Aleppo, where UN officials say the regime is using fighter jets against the rebels.

Human rights monitors say 20,000 people have been killed in the conflict.

The Kremlin said on Wednesday that Putin would strongly defend Russia's position on the crisis, on his first visit to Britain since the G8 summit at Gleneagles, Scotland, in 2005.

"We expect to have another opportunity to continue our dialogue and explain to the British side our well-argued, consistent and absolutely clear position on the (Syria) problem," Putin's spokesman Dmitry Peskov said.

Britain pledged more support for the rebels after Russia and China blocked the latest UN resolution against the Assad regime last month.

Despite tensions over Syria, Cameron hopes to seal major trade deals with Putin before they travel to the ExCel centre in east London, the largest competition venue at the Olympics, to watch the judo.

"My major priority is to get those trade deals, to get that investment -- and not to concentrate on what's happening on the mat," Cameron joked last week.

Putin, 59, has often been filmed in Russia training with top-class judokas as part of his image as a sportsman and lover of rugged pursuits.

On Cameron's last visit to Moscow in 2011, he and Putin's predecessor Dmitry Medvedev -- now Russia's prime minister -- vowed to heal relations, and the pair signed trade deals worth $340 million (280 million euros).

Cameron also met with Putin on that occasion in the first high-level contact between the Russian and any British minister or diplomat since 2007, but the the two countries acknowledged a failure to solve their differences over Litvinenko.

Russia's refusal to extradite Andrei Lugovoi, the chief suspect in the murder who later became a lawmaker, led to a sharp deterioration in ties under the former Labour government in London.

Lugovoi, who is accused of lacing Litvinenko's tea with highly radioactive polonium, denies any involvement in the killing.

Putin's trip to London on Thursday also comes as British rock stars urged the president to give a fair trial to female punk band Pussy Riot, whose three members face up to seven years in jail for singing an anti-Putin song in a Moscow cathedral.

The British stars, including The Who's Pete Townshend, Pulp frontman Jarvis Cocker and Franz Ferdinand singer Alex Kapranos, called the charges against the three female band members "preposterous" in a letter to the Times newspaper.