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However, he said it was too early to determine whether the vote was a success, and he cautioned that the combination of a low turnout in some areas and fraud allegations could threaten the results.

“That may be a toxic combination,” Mr. de Mistura said.

Jed Ober, the head of the U.S.-based observer group Democracy International, said the tallying and fraud-investigation process will be key to determining the election’s validity.

“Right now is a pretty critical time,” Mr. Ober said. “They will be following up on claims and verifying them. So much remains to be seen.”

Last year’s presidential vote was so tainted by ballot-box stuffing and rejiggered tallies — much to Mr. Karzai’s benefit — that nearly a third of his votes were thrown out.

If Afghans don’t accept the results of the vote, it could have a profound effect both inside the country and with Afghanistan‘s international backers, who have 140,000 troops in the country and have spent billions trying to shore up Mr. Karzai’s administration.

Abdullah Abdullah, the runner-up to Mr. Karzai in the 2009 poll, has suggested that there could be unrest if voters feel disenfranchised and that candidates installed despite accusations of fraudulent voting could lead to a rubber-stamp parliament in the hands of the government.

However, an election perceived as legitimate could go some way to building public faith in a democratic system which has struggled to take root since the hardline Taliban regime was ousted in a U.S.-led invasion in 2001. The election drew a wide array of candidates, and at least in key urban centers, campaigning was vigorous and citizens on Saturday voiced resolve in voting despite the threat of militant attack.

Violence continued on Sunday, with three rockets fired a meeting of senior officials in southern Kandahar province which was intended to rally support against the Taliban. The closest landed about 45 yards away from the meeting in Arghandab district, attended by the provincial governor and Mr. Karzai’s brother Ahmad Wali Karzai, who chairs the provincial council. No one was hurt.

In the north, an insurgent rocket attack killed six children in Ali Abad district of Kunduz province, the Interior Ministry said without providing further details.

Meanwhile, NATO forces said they killed seven insurgents in an attack Saturday targeting a Taliban commander at a village compound in volatile Nangarhar province in the east.

Ghafor Khan, the district police spokesman, said five people were killed and two wounded in the attack. He said investigators were determining whether the casualties were insurgents or civilians. NATO said its initial reporting was that no civilians were killed or hurt.

Afghan officials repeatedly have warned that civilian casualties undermine anti-insurgency efforts.

NATO said three of its service members died in attacks in Afghanistan on Saturday. Two died in a bomb attack in the south and another in an insurgent attack in the north. Their nationalities were not disclosed.

Associated Press writers Rahim Faiez and Dusan Stojanovic in Kabul and Mirwais Khan in Kandahar, Afghanistan, contributed to this report.