Afghan president Hamid Karzai's "mental stability" is in doubt and he may even be taking illegal drugs, America's former top diplomat at the United Nations mission to Afghanistan has claimed.

In an interview with a US cable television network Peter Galbraith, the former UN deputy head of mission in Afghanistan described Mr Karzai as "emotional" and "off balance," and called for President Barack Obama vastly to limit his power to appoint officials until he proved himself a reliable partner to the U.S.

"He's prone to tirades. He can be very emotional, act impulsively," Mr Galbraith told MSNBC, adding: "In fact, some of the palace insiders say that he has a certain fondness for some of Afghanistan's most profitable exports."

Asked to back up his claim, he replied: "There are reports to that effect, but whatever the cause is the reality is he can be very emotional."

Mr Galbraith's comments will do nothing to soothe deepening tensions between Afghanistan and the US, with Mr Karzai blaming foreigners for last year’s election fraud, accusing Western troops of meddling in his country’s internal affairs and even threatening to join the Taleban.

Mr Galbraith, who was sacked after a row with his boss Kai Eide over how to respond to the fraud-riddled presidential elections, repeated claims he had previously made, that the fraud benefited the campaign of the incumbent president.

"The fraud was organised by Karzai's appointees and he was the beneficiary of it," he told the cable channel network.

Scoffing at Mr Karzai's threat to join the Taleban, he added: "What possible advantage is it for him to be suggesting he might join the Taleban, to accuse, as he did today, the US of organising the fraud in the election that got him a second term?"

It was at a closed meeting of Afghan parliamentarians in Kandahar on Saturday that the increasingly eccentric President threatened to join the Taleban. He also said that the Taleban could be seen as a legitimate resistance movement. “If I come under foreign pressure, I might join the Taleban,” he told MPs.

But Mr Galbraith dismissed his remarks as "off balance."

"These comments also suggest that he's slightly off balance, at least slightly off balance," he said.

As Mr Galbraith spoke, it appeared the White House was increasingly ready to show the Afghan president a diplomatic cold shoulder.

Speaking of Mr Karzai’s planned visit to Washington on May 12, Robert Gibbs, President Obama’s official spokesman, said that the visit was still on, but added: “We certainly would evaluate whatever . . . further remarks President Karzai makes, as to whether it is constructive to have that meeting.”

Asked if Mr Karzai was an ally of the U.S., Mr Gibbs would only refer to him as the democratically elected leader of Afghanistan

But Mr Galbraith said his "antics" , which showed he was not a "reliable partner" could damage the prospects of success for Mr Obama's troop surge in Afghanistan.

"If our troops cannot accomplish the mission because we don’t have the reliable partner, then why should they be there?" Galbraith said. "It is a waste of very precious military resources."