President Barack Obama warned Iran on Wednesday that the window of opportunity for a diplomatic solution to the stand-off over its nuclear programme was “shrinking” and that Tehran should meet its “international obligations” to come clean over its activities.

At a press conference with David Cameron, the UK prime minister, at the White House, Mr Obama said both the US and UK “are determined to prevent Iran from acquiring a nuclear weapon”.

As six world powers prepared to begin negotiations with Iran over its nuclear programme, he said he had sent a message “personally” to the Iranian leadership that it should re-enter international arms talks in good faith.

But Mr Obama warned: “The window for solving this issue diplomatically is shrinking. I’m determined to prevent Iran from getting a nuclear weapon.”

He added: “Tehran must understand that it cannot escape or evade the choice before it. Meet your international obligations or face the consequences.’’

By issuing his tough message on Iran alongside Mr Cameron, Mr Obama signalled that both the US and UK were prepared to put pressure on the Tehran regime to reach a negotiated deal over its programme in the next few months.

Iran is set to begin talks with six world powers next month. However, Mr Obama said that until now Iran had sought “to delay, to stall”.

The other major focus of their discussion in the White House was on Afghanistan. Here, both leaders also made clear that there would be no deviation from plans to end the combat role for US and UK troops by the end of 2014.

However, the ability of the US and UK to see out their mission along existing plans was again challenged after what appeared to be a possible breach of security at Camp Bastion, the joint US and UK base in Helmand province.

A truck drove on to the runway at the base and burst into flames, according to reports. The incident took place as Leon Panetta, the US defence secretary, was arriving at Camp Bastion.

Both men outlined plans to shift the Nato war effort in Afghanistan toward a more supportive role in 2013 while Afghan forces increasingly take the lead. However, Mr Obama declared: “I do not anticipate that we will make any sudden additional changes to the plan that we currently have.”

Under the administration’s current plan, the existing US force in Afghanistan of about 90,000 troops will be reduced by 23,000 by the end of the summer, with almost all remaining troops expected to leave by 2014.

After a massacre of Afghan civilians by a US soldier at the weekend, there have been renewed calls for a swifter drawdown of troops next year from leading Democrats and even a few Republicans. However, military leaders say a faster pull-out of US or other Nato forces could lead to a sharp increase in instability in the country.

Mr Obama acknowledged there would be conversations with other nations about troop levels for next year before a key Nato summit in Chicago in May. But his comments suggested the US will oppose a swifter-than-planned withdrawal.

The White House discussions also focused on economic growth and trade. Mr Cameron said foreign direct investment between Britain and America was the largest in the world, sustaining about a million jobs. “So deepening trade and investment between us is crucial and can really help to stimulate growth,” he said.

Asked about the differing economic approaches of the UK and the US, Mr Obama said both nations had the same goals. “The United States and Britain are two different economies in two different positions.

“Our objectives are common, which is we want to make sure we have governments that are lean, that are effective, that are efficient, that are providing opportunities to our people, that are properly paid for so that we are not leaving it to the next generation.”

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