A U.S. marine uses his scope to watch the village of Dahaneh in Afghanistan’s Helmand province on Monday. (Julie Jacobson/Associated Press)Three NATO troops — two from Estonia and one from the United States — were killed in attacks in southern Afghanistan as fraud charges continued to pour in Monday from last week's turbulent presidential vote.

The American service member died in an insurgent attack Sunday, the U.S. military said without providing details. Estonia's Defence Ministry said two soldiers were killed after their unit stumbled on a roadside bomb in southern Helmand province.

It was the 37th death for the U.S. military in Afghanistan since the beginning of August, a month that has seen a jump in attacks and violence as the country prepared for its second-ever direct presidential election last week.

Though millions of Afghans went to the polls on Aug. 20, turnout was dampened by Taliban threats leading up to the balloting. Those who voted did so amid rocket attacks in the south, a gunbattle in the capital and fighting that trapped people inside stations in the east.

Allegations of fraud and intimidation have streamed in since election day. The independent Electoral Complaints Commission said Monday it has so far received about 45 complaints that could affect the outcome of the election if proved valid.

Presidential candidates allege vote fraud

The most common was ballot-box stuffing. Both President Hamid Karzai and the leading challenger, former foreign minister Abdullah Abdullah, have accused each other of fraud.

The United Nations urged candidates and the Afghan people to wait for fraud charges to be investigated before doubting the legitimacy of the election.

"There's no doubt that there have been irregularities during the polling day," said Kai Eide, the UN's top official in Afghanistan.

"I appeal to the candidates and to their campaigns, and also to the voters, to demonstrate the patience and calm that is required" while independent monitors investigate allegations, he said.

Eide also responded to reports that the UN had called the election a success by reiterating his statement that it was "an important achievement" for the people of Afghanistan.

"It is too early to use words that go beyond that," Eide said.

U.S. commanders predicted a deadly summer after President Barack Obama ordered 21,000 more U.S. troops to Afghanistan to turn the tide against a resurgent Taliban and shift the focus in the fight against Islamist extremism from Iraq.

U.S. death toll 44, 6 Estonian troops killed

July was the deadliest month for American forces in Afghanistan since the beginning of the conflict, with 44 dead. Estonia has 289 soldiers in Afghanistan and the recent deaths mean the small European country has lost six soldiers there.

With the security situation in Afghanistan appearing increasingly difficult, many are suggesting that U.S. commanders may request thousands more troops.

On Sunday, Adm. Mike Mullen, chair of the Joint Chiefs of Staff, described the situation in Afghanistan as "serious and deteriorating," but refused to say whether additional forces would be needed.

"Afghanistan is very vulnerable in terms of [the] Taliban and extremists taking over again, and I don't think that threat's going to go away," he said on CNN's State of the Union.