President Hamid Karzai extended his lead over his top challenger in Afghanistan's presidential election, new vote results showed Wednesday, but remains short of the 50 percent threshold that would allow him to avoid a two-man runoff.

Afghan election officials are slowly releasing results from last week's presidential election, and final certified results will not be ready until at least mid-September, after dozens of serious complaints of fraud have been investigated.

Low voter turnout and the fraud allegations have cast a pall over the vote, seen as critical to efforts to stabilize the country, which is wracked by Taliban insurgents and doubts over its fragile democracy. Top challenger Abdullah Abdullah has accused Karzai of widespread rigging, including ballot stuffing and voter intimidation, claims Karzai's camp has denied.

The latest returns boost Karzai's standing to 44.8 percent. Abdullah, a former foreign minister, now has 35.1 percent. The count is based on returns from 17 percent of polling stations nationwide, meaning the results could still change dramatically. Tuesday's returns had Abdullah trailing Karzai by just 3 percent.

Although millions of Afghans voted last Thursday, apathy and fear of militant attacks meant turnout was down on the nation's first direct presidential election in 2004 that was swept by Karzai.

This summer has been Afghanistan's most violent since the 2001 U.S. invasion. President Barack Obama ordered an additional 21,000 troops to the country this year, in part to help secure the elections. But violence has continued to rise.

NATO said two U.S. troops died Wednesday in two separate attacks, keeping August on pace to be the deadliest month of the war for the U.S. military. The two deaths bring to 43 the number of U.S. troops killed this month. Last month was the deadliest of the war, when 45 U.S. troops died.

On Tuesday night, a huge bombing hit the main southern city of Kandahar, killing at least 43 people and wounded 65. The attack, which destroyed dozens of buildings, took place in a district that includes U.N. facilities and an Afghan intelligence office. Rescue workers were still pulling injured people from the ruins early Wednesday.

An Afghan employee of the International Committee of the Red Cross was among the dead.

Most of the victims were ordinary Afghan civilians, like Mohammad Masoom, 26, a tailor whose shop was near the blast site. His father,Mohammad Yusuf,discovered his son's body at hospital and condemned the "cruel people" behind the bombing. His other son was wounded in the blast.

"They killed my son, and so many innocent people. I don't know what these people want," Yusuf said, weeping.

The Interior Ministry said the blast was from remote-controlled explosives planted in a truck, although local officials had said earlier a cluster of five vehicle bombs caused the blast. The attack came just two hours after the first batch of election results were released, although there was no immediate indication it was timed to coincide with the announcement.

The Taliban on Wednesday denied any responsibility for the attack in Kandahar — the Islamist movement's spiritual home. Iy often denies involvement in attacks that kill civilians.

"We are denying responsibility, and condemn this attack in which innocent civilians were killed," Taliban spokesman Qari Yousef Ahmadi wrote in a text message sent to an Associated Press reporter.

Kandahar Gov. Tooryalai Wesa said officials could not yet say who was responsible. The attackers "have no conscience or families," he said.

Interior Minister Hanif Atmar said Afghan security forces, supported by NATO troops, would conduct a search operation and try to have suspects in custody within 72 hours.

Kandahar lies in a swath of southern Afghanistan dominated by Pashtuns, the country's largest ethnic group from which the Taliban derives its support and recruits.

But it is also a crucial electoral battleground, particularly for Karzai, who as a Pashtun and a Kandahar native, is expected to perform well there.

The presidential returns announced Wednesday are based on partial results from 28 of the 34 provinces, but few votes have been counted from Kandahar and no votes from neighboring Helmand province, two areas that would boost Karzai's totals.

An opinion poll taken in July showed 44 percent of Afghans would vote for Karzai, and 26 percent for Abdullah. The poll of 2,400 people was funded by the International Republican Institute, a non-governmental organization that receives funding from U.S. government aid arm USAID. The poll had a margin of error of plus or minus 2 percentage points

Ramazan Bashardost — a candidate likely to come in third in the election — stood up at Wednesday's news conference and told officials they were breaking the law by announcing results before the Electoral Complaints Commission carries out its work.

The head of the Independent Election Commission, Daoud Ali Najafi, said all procedures were in accordance with the law, which stipulates that final "certified" results cannot be released until the complaints commission finishes its work.

The complaints commission has received more than 1,400 complaints, more than 150 of which could affect the vote's outcome, said spokeswoman Nellika Little.