U.S. President Barack Obama and British Prime Minister David Cameron have renewed calls on Russia and China to back the United Nations' calls to condemn the regime of Syrian President Bashar al-Assad.

The pair urged support for UN special envoy Kofi Annan in his bid to gather full Security Council backing for a resolution denouncing the crackdown by Syrian government forces on the opposition movement.

The joint statement came amidst a three-day state visit by Cameron, who has so far been treated to a ride on Air Force One, a March Madness college basketball game and a formal state dinner at the White House on Wednesday night.

Both Russia and China have vetoed two previous efforts by the Security Council to gather support for a resolution, saying that opposition forces must also be called upon to observe a ceasefire.

Speaking at a press conference Wednesday, Cameron said Russia could be persuaded to go along with the international community after watching the bloodied, broken, brutal regime butchering people nightly on the television screens, USA Today reported.

It's going to take a lot of hard work, it's going to take a lot of patient diplomacy, but I think it's actually in Russia's interests that we deal with this problem, he added.

Cameron and Obama also discussed the crisis surrounding Iran's nuclear program.

Obama also warned that the window for solving this issue diplomatically is shrinking, adding to previous threats that military action remains a possibility.

The two leaders also discussed the drawn down of troops from Afghanistan, reaffirming their commitment to NATO's strategy for a transition to Afghan security responsibility in 2013 and withdrawal of combat troops by the end of 2014.

In terms of pace, I don't anticipate at this stage that we're going to be making any sudden additional changes to the plan that we currently have, Obama said, according to Reuters.

The press conference brought to a close an overtly symbolic three-day state visit designed to reassure the British press and public that the special relationship between the two nations is alive and well.

After a 19-gun salute and a marching band, the pair praised the historic bonds between the U.S. and UK, which became noticeably strained under Cameron's predecessor, Gordon Brown.

While in the past overt friendships between the British Prime Minister and U.S. President have boosted poll ratings on both sides of the Atlantic, Cameron will be all too aware of the negative publicity faced by previous incumbent Tony Blair.

The former Labour party leader suffered a drubbing by the British press after being labelled a lap dog of Gerorge W Bush for his willing support of the wars in Afghanistan and Iraq.