The US ambassador to Afghanistan has dramatically intervened in the debate about troop reinforcements, warning President Obama against committing tens of thousands of extra troops to the country.

Karl Eikenberry, a retired army general who commanded US forces in Afghanistan from 2005-2007, detailed his concerns in two classified cables last week.

Yesterday President Obama questioned Mr Eikenberry about his views by video-link during a meeting of his White House war cabinet, as he continues his lengthy deliberations on the question of troop numbers for Afghanistan.

Mr Eikenberry's concerns reportedly focused on the behaviour of Hamid Karzai, the Afghan president recently re-elected for a five-year term in a poll tainted by allegations of systematic fraud. He is said to have questioned Mr Karzai's suitability as a long-term strategic partner, because of widespread corruption in his first administration and the presence of warlords and drugs smugglers in positions of influence.

The warnings are being seen by analysts as a shot across Mr Karzai's bows, as he puts together his government before he is inaugurated for his second term on November 19.

The timing and content of Mr Eikenberry's intervention has reportedly infuriated General Stanley McChrystal, the Nato and US commander in Afghanistan, who had asked for an extra 40,000 troops to avert a looming military defeat.

They also prompted a quick reaction from the Secretary-General of Nato, Anders Fogh Rasmussen, who insisted today that Nato backed the need for more troops despite Mr Eikenberry's reservations.

Speaking after a meeting in Downing Street with Gordon Brown, Mr Rasmussen was asked what advice he would give Mr Obama.

He replied: "We are right now in an intense phase of consultation among allies and I expect a decision on troop numbers to be taken within a very few weeks so I think it is a bit premature to make any final judgment on troop numbers. Basically I share General McChrystal’s view, his assessment, his recommendation of a broad counter-insurgency strategy.

He added: "I have not made a final decision on the exact troop numbers but for sure we need to strengthen training and education of Afghan soldiers and Afghan police so we will definitely need more trainers, more education facilities, equipment and money to sustain an increased number of Afghan security forces."

A White House statement read out after yesterday's session was carefully worded to capture the administration's ambivalence about expanding its military commitment with a week and corrupt government as its partner in Kabul.

“The President believes that we need to make clear to the Afghan Government that our commitment is not open-ended,” a White House official told journalists. “After years of substantial investments by the American people, governance in Afghanistan must improve in a reasonable period of time to ensure a successful transition to our Afghan partner."

The ambassador's intervention, reported today in The Washington Post, comes as draft lists circulated in Afghanistan of names of those said to be in line for posts on Mr Karzai's government. It is understood many of those listed are tainted by corruption allegations and are considered unacceptable by western governments.

Four options appear to have emerged on future troop reinforcements, but despite various high-level leaks it is far from clear which one Mr Obama will eventually embrace.

They range from a low-end option involving involving the deployment of about 10,000 to 15,000 US troops, mostly trainers to accelerate the expansion of the Afghan security forces, although such a move is not considered cost-effective by Pentagon planners.

The most ambitious option involves giving General McChrystal the 40,000 troops. A third course involves sending around 30,000 to 35000 and urging Nato partners to make up the shortfall.

The final option on the table, dubbed the "hybrid" option, would involve a surge of around 20,000 troops to reinforce major population areas coupled with a renewed counter-terrorist effort against al-Qaeda and Taleban targets in lawless tribal areas along the border with Pakistan.

Mr Obama leaves today on his first presidential tour of Asia, putting off for at least another nine days a decision that his critics say should have already been made. His press secretary says a decision is still weeks away.

Yesterday's war cabinet meeting in the White House situation room was the eighth since August specifically focusing on troop reinforcements. Participants included the Defence Secretary Robert Gates, the Secretary of State Hillary Clinton, Vice President Joe Biden and Admiral Mike Mullen, Chairman of the Joint Chiefs of Staff.

Mr Eikenberry and General McChrystal both participated by video link from Afghanistan.

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