enlarge The Associated Press

Afghan President Hamid Karzai, left, talks with U.N. Secretary-General Ban Ki-moon in Kabul, Afghanistan, Monday, as the U.N. chief made a surprise visit to the country.

KABUL — Election officials declared Afghan President Hamid Karzai the winner of a new five-year term Monday, canceling Saturday's runoff election just one day after Karzai's sole challenger quit the race. The decision ended weeks of political drift since a first presidential poll in August was found invalid because of massive fraud.

In the capital, a sense of relief was instant and palpable. Kabul residents honked horns and exchanged celebratory text messages as the news spread. American, European and U.N. officials rushed to congratulate Karzai and pledged to work closely with his new administration.

But the decision to allow Karzai to begin a new term without a clear mandate raised questions about the legitimacy of his future administration. President Obama on Monday called for a new chapter in Afghanistan relations. Obama, who is weighing the future of U.S. policy in Afghanistan, now in its ninth year of war, said he congratulated Karzai in a phone call Monday.

"Although the process was messy, I am pleased to say that the final outcome was determined in accordance with Afghan law," Obama said.

The issue of Karzai's legitimacy goes to the heart of the U.S. relationship with Afghanistan. Obama is considering a military request for another 40,000 U.S. troops to fight there. Currently, there are 68,000 U.S. troops authorized in the country; NATO has sent about 40,000 more.

"This has to be a point in time when we begin to write a new chapter based on improved governance, a much more serious effort to eradicate corruption and joint efforts to accelerate training of forces," Obama said.

"The proof is not going to be in words but in deeds. We are looking forward to consulting closely with his government in the weeks and months" to come and to assure that the "Afghan people are seeing progress on the ground," Obama said.

No date for announcing Obama's decision on troop expansion has been set.

Despite calls for calm by Karzai's rival, Abdullah Abdullah, there were fears that opposition supporters might cause violent unrest.

In a potentially worrisome development Monday evening, Abdurrashid Dostum, a former ethnic militia leader and political ally of Karzai who has a long track record of human rights abuses, arrived on an international flight at the Kabul airport. Dostum, who has been living in exile in Turkey, is a longtime rival of a northern strongman who backed Abdullah.

An official from Dostum's political party confirmed the ethnic Uzbek commander's arrival, but insisted that the visit was "normal" and that Dostum had no special agenda. Dostum's last known visit here, to support Karzai's campaign in August, lasted only a few days after U.S. officials complained.

Supporters of Dostum in northern Faryab province, reached by phone Monday night, said they expected him to visit there to rouse support in case of violence from followers of Attah Mohammed, the governor of nearby Balkh province. Mohammed, who abandoned Karzai to back Abdullah, is a former militia leader and longtime rival of Dostum; the two camps have at times fought bloody battles. Mohammed has said he will not recognize a new Karzai government.

The terse announcement of Karzai's victory was made by the chairman of the Afghan Independent Election Commission, Azizullah Lodin, whose removal had been demanded by Abdullah as one of several conditions for remaining in the race. After Karzai rejected the demands, Abdullah, a former foreign minister and eye doctor, withdrew Sunday, saying he did not believe the runoff would be fair or transparent.

Lodin said the seven-member panel had been "fully prepared" to hold the runoff but had decided that it should be canceled for a combination of reasons. He noted that there was only one candidate, that the poll would be costly and dangerous to hold, and that it could have created "many challenges...