President Karzai has six months to sideline his brother and reduce corruption or risk losing American support, Afghan officials have told The Times.

Senior palace insiders said that President Obama delivered the ultimatum when he congratulated Mr Karzai on his re-election on Monday. Top of his demands was action against corruption, the appointment of “reform-minded ministers” and several high-profile scalps to prove Mr Karzai’s commitment to cleaning up his Government.

“If he doesn’t meet the conditions within six months, Obama has told him America will pull out,” said an official with access to Mr Karzai’s inner circle. “Obama said they don’t want their soldiers’ lives wasted for nothing. They want changes in Cabinet, and changes in his personal staff.”

It is extremely unlikely that British troops would stay in Afghanistan if US forces were withdrawn.

The President’s half-brother, Ahmed Wali Karzai, has repeatedly denied claims that he controls Afghanistan’s billion-dollar heroin trade. As head of Kandahar’s provincial council, he is the main powerbroker in the south of the country, but the President has refused to remove him, insisting that there is no proof of wrongdoing.

In his acceptance speech yesterday, Mr Karzai vowed to eradicate the “dark stain of corruption”, which he admitted had undermined faith in his regime. But Mr Obama had earlier cautioned: “The proof is not going to be in words; it’s going to be in deeds.”

Afghan officials said that there were efforts to find Wali Karzai a new position. “There have been talks to find a new term of reference for the President’s brother,” said one. One option would be to send him abroad.

The American Embassy is understood to have warned Mr Karzai it will start collecting evidence against Wali Karzai if he is not removed from Kandahar. “They want tangible progress over the next three to six months on corruption, the culture of impunity and rising crime,” said a senior Western analyst with close links to the State Department.

“They have told Karzai they are going to start collecting evidence and if he doesn’t act on it, they will go public. We’ve never been able to back up our claims about his brother, or any of the commanders, with evidence. Now when we say something, we’ll show the smoking gun and say, ‘Arrest him’.”

An embassy official said Washington wanted proof that Mr Karzai was taking the challenges “as seriously as we are”. Last week Wali Karzai denied fresh allegations that he has been on the CIA payroll for much of the past eight years. The claims, in The New York Times, prompted the Republican Senator John McCain to demand he be sent into exile.

The allegations have also exposed tensions between the State Department, which wants people like Wali Karzai removed, and the secret intelligence agencies that rely on morally dubious partners to get things done.

Wali Karzai, who campaigned for his brother in Kandahar, celebrated their victory yesterday with a feast for a thousand people at his home.

Diplomats said the milestones for Mr Karzai’s progress would be agreed at a conference in Kabul immediately after his inauguration. A second conference, six months later, will be convened to measure achievement. If Mr Karzai doesn’t meet his targets, several options are being considered, including scaling back the military presence.

Mr Obama is due to make a decision on whether to send up to 40,000 more troops in the coming weeks. General Stanley McChrystal, the US commander in Afghanistan, requested reinforcements to mount a counter-insurgency strategy. Others in the White House, including Joe Biden, the Vice-President, favour using foreign forces and relying on unmanned drones and Special Forces raids to target terrorist training camps.

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