Afghan president Hamid Karzai's "mental stability" is in doubt and he may even be taking illegal drugs, a former top UN envoy to the war-torn country told MSNBC television.

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

Asked to back up his claim, the former UN deputy head of mission in Afghanistan replied: "There are reports to that effect, but whatever the cause is the reality is he can be very emotional."

Mr Galbraith's comments came amid a deepening feud with the US that saw him reportedly charge that Washington was interfering with Afghan affairs and that foreigners helped rig last year's elections.

"The fraud was organised by Karzai's appointees and he was the beneficiary of it," said Mr Galbraith, an American who was fired by the UN in a dispute over how far to press Afghan authorities on electoral fraud charges.

"This continued tirade raises questions about his mental stability and frankly this has been something that has been of concern to diplomats in Kabul."

The Wall Street Journal reported on Monday that Mr Karzai had told politicians that the US was interfering with Afghan affairs and that the Taliban would become a legitimate resistance movement if it did not stop.

The paper said that in the private meeting, the Afghan president even suggested he could join the Taliban if parliament did not support his efforts to take control of the country's election commission.

"What possible advantage is it for him to be suggesting he might join the Taliban, to accuse, as he did today, the US of organising the fraud in the election that got him a second term," Mr Galbraith said.

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

Mr Galbraith warned Mr Karzai's "antics" could harm the prospects for success of US president Barack Obama's troop surge in Afghanistan because it was clear the Afghan leader "cannot be a reliable partner".

- AFP