He told four visiting U.S. congressmen Thursday that setting a pullout date would make the Taliban believe they could simply hold out until the U.S. and its NATO allies had left.
Karzai also said more should be done to shut down Taliban sanctuaries in Pakistan, saying the war could not be won as long as insurgents could seek refuge there.
In a statement from his office, according to various media reports, Karzai said much progress had been made in rebuilding Afghanistan after decades of war, but that civilian deaths caused by NATO operations had held back the war on the insurgents.
Pakistan has launched a number of military operations against the Taliban and other militant groups but says the deadly monsoon floods that have caused widespread destruction throughout the county have hampered its efforts.
Karzai's national security officer, Rangin Dadfar Spanta, told the BBC that fighting the Taliban in Afghanistan alone would not be enough.
"Even if we defeat the Taliban, al-Qaida and other terrorist groups within Afghanistan, (if) we don't destroy the recruitment center, we don't destroy the causes, it is not possible to win the war," he said.
Spanta has often accused Pakistan of providing sanctuary to militant groups and says Islamabad regards Afghanistan as coming under its influence, the BBC said.
Karzai's opposition to the pullback ordered by President Barack Obama is widely supported throughout the region, according to retired U.S. Army Gen. Jack Keane.
"Just the policy itself has done damage in the region because it has clearly encouraged our adversaries and put skepticism in the minds of our friends," Keane told Al-Jazeera.
The plan had made it harder for the U.S. even if the policy was conditional, "and even if there was very little withdrawal in July," he was quoted as saying.
According to Gen. David Petraeus, the commander of NATO forces in Afghanistan, the July 2011 date announced by the White House was not "when U.S. troops begin an exodus." Rather, he said, it was simply the beginning of a process in which more responsibilities would be handed over to Afghan forces.




