Afghanistan's election commission canceled Saturday's presidential runoff and proclaimed President Hamid Karzai the victor of the war-ravaged nation's tumultuous ballot for another five-year term.

Independent Election Commission chairman Azizullah Lodin announced Karzai as the victor during a news conference in Kabul on Monday.

President Hamid Karzai had already effectively secured a second term when his only challenger, former Foreign Minister Abdullah Abdullah, dropped out of the race Sunday because he said the vote would not be free or fair. The first round was marred by massive fraud.

VIDEO: Interview With Former Challenger Abdullah

The United States and its allies congratulated President Hamid Karzai on winning a second term Monday.

The U.S. embassy said in a statement that it looked forward to working with Karzai to support reform and improve security.

The British government and the United Nations also issued statements of congratulations.

Meanwhile, the U.N. chief made a surprise visit to Afghanistan on Monday as international pressure mounted for a quick resolution to the country's electoral turmoil following the withdrawal of President Hamid Karzai's only challenger.

U.N. Secretary-General Ban Ki-moon was to meet with the two men "to assure them and the Afghan people of the continuing support of the United Nations toward the development of the country," the statement said.

A deadly attack last week on a Kabul guest house where U.N. election workers were staying has raised questions about whether the U.N. might scale back in Afghanistan. Militants stormed the compound before dawn, killing five U.N. staffers and three Afghans.

The U.N. kept operating after an August 2003 truck bombing at its headquarters in Baghdad, which killed 22 people, including mission chief Sergio Vieira de Mello, but after a second bombing it shut down operations in Iraq in late October 2003 for years.

Ban will also meet with U.N. staff and security officials, the statement said.

Taliban threats of more violence and the difficulty of organizing and securing the balloting mean that officials are likely looking for a way to end the process without sending people back to the polls this week.

Karzai has said the runoff should go forward as planned, but there is no clear article in Afghanistan's constitution or electoral law to address the situation. The chairman of the Independent Election Commission, Azizullah Lodin, said Sunday that he would have to meet with constitutional lawyers before deciding how to proceed.

Repeated calls to election officials on Monday were not returned.

It has been more than a month since the Aug. 20 balloting that aimed to strengthen the Afghan government but instead undermined its credibility both at home and with key allies like the United States.

The vote was characterized by rampant ballot-box stuffing, and fraud investigators threw out nearly a third of Karzai's votes. That move dropped Karzai below the 50 percent threshold needed to win outright, forcing the runoff vote.

A bevy of international figures, including U.S. Sen. John Kerry, U.S. Secretary of State Hillary Clinton and British Prime Minister Gordon Brown, were involved in convincing Karzai to accept the runoff vote.

U.N. and U.S. representatives were still involved in negotiations with the two about a power-sharing deal as recently as Sunday morning, according to a Western diplomat who was familiar with the talks but spoke anonymously because of the sensitivity of the discussions.

The U.S. and the U.N. have both issued statements calling for a quick resolution now that Abdullah has bowed out.

Abdullah has said his decision not to participate in the runoff is final. But in a sign of how much the situation is in flux, a spokesman said Monday that they could be open to still having a second round if it is delayed to put in safeguards to prevent fraud.

"Lots of opportunities have been missed and election day is very close," Fazel Sancharaki said. "If President Karzai accepts Dr. Abdullah's conditions we are thinking of a second date for the election."

The Associated Press contributed to this report.