Rising death tolls and the political crisis brought on by a fraud-marred election have prompted the Obama administration to review its entire Afghanistan war strategy.
Two of the U.S. service members were killed instantly in the Thursday blast and two others died of their wounds, the U.S. said in a statement. No further details were released.
The deaths bring to 25 the number of American service members killed in Afghanistan this month, according to an Associated Press count.
Elsewhere, four Afghans, including at least two civilians, died during a firefight Friday between militants and a joint international-Afghan force in Ghazni province. There were conflicting accounts of the gunbattle.
The NATO-led coalition said two militants fired from a two-story building and troops returned fire, killing a pair of gunmen. "When the joint force entered the building, they discovered two civilians who subsequently died from their wounds," the coalition said in a statement. "It is unclear if the enemy militants or the joint force are responsible for the deaths."
Ghazni police chief Gen. Khail Buz Sherzai said the dead were all civilians from the same family. A native of Mangor village, Mohammad Sarwar, said the operation began late Thursday when U.S. and Afghan forces raided several houses overnight, blowing apart doors and window with explosives. He also said four civilians were killed in the operation and several were beaten.
Insurgent violence has increased across Afghanistan this year, coinciding with a boost in U.S. military numbers. President Barack Obama is now considering whether to commit still more American troops to the about 65,000 already here.
The White House is considering various options, including a sharp increase in the number of U.S. troops or shifting the focus to missile strikes and special operations raids against al-Qaida members hiding in neighboring Pakistan.
Obama is not expected to decide until after the Afghans determine whether they must hold a runoff election between President Hamid Karzai and his top challenger, Abdullah Adbullah.
Preliminary results from the Aug. 20 poll had put Karzai in the lead with 54.6 percent of the vote compared to about 28 percent for Abdullah. The fraud rulings could eliminate enough Karzai votes to push him below the 50 percent threshold to force a second round.
A spokeswoman for the Electoral Complaints Commission said the panel has completed the bulk of its investigation but commissioners are still analyzing complaints and calculating figures before deciding on a runoff.
Investigators late Thursday completed an audit of 3,377 polling stations that returned unlikely results showing 100 percent turnout or a single candidate receiving 95 percent of the vote, said Nellika Little, a commission spokeswoman.
But the panel is still investigating individual fraud complaints. "We are still working on the numbers," Little told The Associated Press. "We haven't figured out a percentage."
An announcement could come at any time, possibly as early as Friday night. Once the country's Independent Election Commission confirms the new tallies, a runoff is supposed to be held within two weeks. But many fear winter snows and insecurity could make the vote difficult or impossible.
In Washington, Karzai's ambassador to the United States, Said Tayeb Jawad, said Thursday that a runoff vote was very likely. He was the first official from Karzai's government to predict publicly that the challenger, Abdullah Abdullah, will have enough support to force a runoff.
Jawad said all sides should work hard to hold the runoff vote swiftly - ideally within a month.
A two-week deadline mandated in the country's constitution is "impossible," Jawad said. He worried that if the deadline slipped far into November, the weather will be too cold in parts of the country. Voters in Afghanistan, a country of great distances and few roads, often must travel long distances and spend significant time outdoors.
Jawad spoke at the U.S. Institute of Peace, and afterward with The Associated Press.
Citing anonymous sources it said were familiar with the results, The Washington Post reported Friday that Karzai's share of the vote had dropped to 47 percent. Little disputed that report, saying the commission's decisions have not been released.
Uncertainty over the election outcome has eaten away at Karzai's legitimacy, leaving Afghanistan in limbo as the Taliban-led insurgency in the countryside deepens and the Obama administration debates its strategy in a war that has become increasingly unpopular in the U.S.
Associated Press Writer Rahim Faiez contributed to this report.