At the same time, international diplomats were urging the top two finishers in the August election to consider a power-sharing arrangement to avoid a costly and divisive runoff.
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.
"Unfortunately, Afghanistan remains a dangerous place. We do try to take all the precautions that we can and we are regularly readjusting our forces to better protect the civilian population here and unfortunately that places our forces at risk," U.S. military spokesman Col. Wayne Shanks said.
Elsewhere, four Afghans, including at least two civilians, died during a firefight Friday between militants and a joint U.S.-Afghan force in Ghazni province. There were conflicting accounts of the battle.
The NATO-led coalition said two militants fired from a two-story building and troops shot back, killing two 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.
Insurgent violence has increased across Afghanistan this year, coinciding with a boost in U.S. military strength - now at more than 65,000.
Obama is not expected to decide on a new strategy until 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 is widely expected as early as Saturday. Then the country's Independent Election Commission will announce final results and say whether a runoff is needed. Any runoff is supposed to be held within two weeks, but many fear winter snows and insecurity could make the vote difficult or impossible.
A Western official said both Karzai and Abdullah were under international pressure to accept some sort of coalition or unity government to avoid a runoff.
The official, who spoke on condition of anonymity because of the sensitivity of the discussions, said the two appeared open to the idea but there were major differences over timing, details and whether such an arrangement was in line with the Afghan constitution.
A former U.S. ambassador to Afghanistan, Zalmay Khalilzad, has been meeting with both Karzai and Abdullah this week, encouraging them to find way out of the political crisis.
"I urged them to recognize the gravity of the circumstances and to rise to the occasion," Khalilzad said.
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