KABUL, Afghanistan – A soldier from the U.S.-trained Afghan army apparently turned his weapon on American troops in volatile southern Afghanistan, killing at least two U.S. soldiers, NATO officials said Saturday.

The incident is the latest that calls into question the allegiances of at least some members of the Afghan security forces, which the Obama administration hopes will be the key to an eventual withdrawal of roughly 100,000 U.S. troops from Afghanistan.

A NATO official, speaking on condition of anonymity because the matter is still under investigation, said the shootings took place late Thursday in southern Helmand province. The area has been the site of intense fighting between the Taliban-led insurgency and thousands of additional troops President Obama sent to Afghanistan this year.

Few other details of the clash were immediately available. CNN quoted a Taliban statement from a militant website as saying the Afghan soldier shot the U.S. troops on an American base in Helmand’s Sangin district.

Just last week, a squad of Afghanistan’s national police in Ghazni province, southwest of Kabul, were reported to have crossed sides to the Taliban, although the details of precisely what occurred are murky.

The International Security Assistance Force said two other allied soldiers died from insurgent attacks Saturday, one in the country’s south and one in the east.

The violence came as a group of parliamentary candidates from across Afghanistan demanded a re-run of September’s elections, saying they’d been wrongfully disqualified, and promised new rallies to protest electoral fraud.

The move by a dozen candidates drawn from provinces in northern and central Afghanistan appeared unlikely to change the outcome of the deeply flawed Sept. 18 election, whose final results have yet to be announced. But it underscored the difficulty of achieving the U.S. goal of building a stable Afghan government.

The elections were marred by charges of vote-rigging, ballot-stuffing, intimidation and other problems. Afghanistan’s electoral body threw out 1.3 million of a reported 5.6 million ballots cast, without a clear explanation, and final results, originally due Oct. 30, have been delayed until at least late November.

A report by the Electoral Complaints Commission, dated Oct. 31 and obtained by McClatchy, shows there have been more than 5,200 complaints of irregularities nationwide

“Your money, your effort, your blood, your sweat and your tears are spent in this country,” said Dawood Sultanzoi, a parliamentarian from Ghazni, who stands to lose his seat — unfairly, he said. Results from Ghazni and other areas were strongly titled against ethnic Pashtuns like Sultanzoi.

“This will drive hundreds of thousands of people to the mountains to join the (Taliban) opposition,” he said.