President Barack Obama said the U.S.-led forces must "focus on finishing the job" in Afghanistan after what he said appeared to have been a successful election in the country, Reuters reported.
Millions of Afghans went to the polls on Thursday, defying Taliban threats of violence and sporadic attacks to choose a president in the midst of a worsening war.
"We had what appears to be a successful election in Afghanistan, despite the Taliban's efforts to disrupt it," Obama said from the White House. "We have to focus on finishing the job in Afghanistan but it is going to take some time."
The election was a test for Obama's new strategy aimed at reversing Taliban gains. U.S. combat casualties have risen amid a U.S. troop buildup and opinion polls have shown weakening American backing for the war.
Support for the Afghan campaign among some NATO allies is also on the wane and the German government scrambled to quash a public debate about pulling German troops out of Afghanistan that has grown louder amid the surge in violence.
The White House said Afghans had turned out to vote in large numbers despite threats of violence and U.S. policy in the eight-year-old war would not change.
Preliminary results are not due for two weeks, although polling stations could begin to report sooner.
The Afghan government said nine civilians and 14 members of the security forces were killed in 135 incidents countrywide on polling day.
Pre-election polls showed Karzai, in power since 2001, was likely to win but not by enough to avoid a run-off against his main challenger, his former foreign minister Abdullah Abdullah, who ran a surprisingly energetic campaign.
If Karzai fails to win more than 50 percent of the vote he will face a run-off in October, most likely against Abdullah.
The election is in large measure a referendum on Karzai, a master coalition builder who is personally liked by most Afghans but also widely blamed for running a government that is corrupt, ineffective and entirely dependent on international aid.
The president relied for votes on the endorsements of many of the country's notorious former militia chiefs, raising alarm among his Western backers that the cost of a victory in the election could be a return of warlords to power.