Senior Karzai insiders told The Times of London U.S. President Barack Obama set the deadline while congratulating Karzai on his re-election to a second term after rival Abdullah Abdullah withdrew from a runoff. Obama's conditions included cracking down on corruption and appointing reform-minded ministers, the report said.
"If he (Karzai) doesn't meet the conditions within six months, Obama has told him America will pull out," one official told The Times. "Obama said they don't want their soldiers' lives wasted for nothing. They want changes in Cabinet, and changes in his personal staff."
The Times said if U.S. forces leave Afghanistan British troops would likely follow.
Karzai has refused to remove his half-brother, Ahmed Wali Karzai, a powerbroker in south Afghanistan, saying there is no proof he controls the country's lucrative heroin trade.
The report said American diplomats in Afghanistan have told Karzai they would start collecting evidence against Wali Karzai if he is removed from his power base. Afghan officials said the government was trying to find Wali Karzai a new position including sending him abroad.
Karzai in an address to the nation Tuesday promised to remove the "dark stain of corruption" from his government.