Spokesman Robert Gibbs also said that Americans should "take heart" that the laws and institutions of Afghanistan had prevailed.

Gibbs further said that there was no reason to believe the Afghan people would not accept a Karzai government as legitimate, even though challenger Abdullah Abdullah, Karzai's former foreign minister, dropped out of a planned run-off vote on Saturday.

The tumultuous election process has coincided with a prolonged study by Obama to decide on a new Afghan strategy, including a request by his top commander, Gen. Stanley McChrystal, for as many as 80,000 additional forces to battle the militant Taliban and its al-Qaida allies.

Only moments earlier Obama declined to comment on a ruling by Afghanistan's election commission to name Karzai the victor.

The results of the initial August vote were overturned last month, with about 1 million ballots marked for Karzai having been thrown out as fraudulent. That left him with fewer than 50 percent, requiring a run-off vote that was set for Saturday. On Sunday, Abdullah, the second-place finisher, dropped out of the race.

Karzai's administration has been under heavy U.S. pressure to eliminate corruption and to widen its support among the country's key ethnic groups.

With Karzai's re-election confirmed, Gibbs said, "now begin the hard conversations about ensuring credibility, ensuring and improving governance, addressing corruption. We are focused on what has to happen in order to have a credible partner."