Afghanistan's election commission has cancelled Saturday's presidential runoff and proclaimed President Hamid Karzai victor of the war-ravaged nation's tumultuous ballot.

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

Karzai's challenger, former Foreign Minister Abdullah Abdullah, dropped out of the race on Sunday because he said the vote would not be free or fair following fraud-marred first round.

Earlier, the UN Secretary-General Ban Ki-moon said world body's work will continue in Afghanistan despite the slayings of UN staff in Kabul last week.

Ban told reporters Monday in the capital that "we cannot be deterred, we must not be deterred. ... The work of the United Nations will continue."

Last Wednesday, insurgents stormed a guest house in Kabul where UN election workers were staying, killing five U.N. staffers and three Afghans.

On Monday, the world body announced it would suspend long-term development work in areas of Pakistan along the Afghan border because of violence.