An Afghan interpreter also was wounded by the 3 p.m. explosion near the city of Ghazni, 60 miles southwest of the capital, Kabul. The soldiers were working around a weapons cache when the blast happened.
Centcom spokesman Capt. Bruce Frame said the cause had yet to be determined in the blast, one of the most damaging blows to American troops since they began battling the Taliban regime more than two years ago.
The Taliban were quickly driven from power but sporadic fighting has continued. Earlier this month, the U.S. death toll reached 100. Sixteen of those deaths occurred in combat including seven when two helicopters took enemy fire March 4, 2002.
The toll includes deaths in other areas of Operation Enduring Freedom, such as a helicopter crash in the Philippines nearly two years ago that killed 10 American soldiers the deadliest in the operation. Seven soldiers were also killed on Jan. 9, 2002 when their tanker plane slammed into a mountain in Pakistan.
The United States provides 9,000 of the 11,000-member coalition troops stationed in Afghanistan. The Army is preparing a spring offensive against Taliban and al-Qaida holdouts amid concern that operations in Afghanistan are not as effective in breaking up terrorist networks as they had hoped.
Separately, investigators sifted through evidence Thursday from suicide bombings that killed British and Canadian soldiers in Kabul the two previous days. The ousted Taliban regime has claimed responsibility for both blasts.
The cause of Thursday's weapons cache blast was not immediately known. The wounded soldiers were evacuated to a hospital at Bagram Air Base, headquarters for U.S.-led coalition in Afghanistan.
The names of the victims were being withheld pending notification of relatives.
Troops at Camp Sootier, the British base in eastern Kabul, held a memorial ceremony Thursday for the soldier killed the day before by a suicide bomber. The victim was identified as Pvt. Jonathan Kitulagoda, 23, from Plymouth in southwest England.
Commanders and diplomats joined about 150 soldiers to hear readings, prayers and tributes from Kitulagoda's friends in a private gathering, said Capt. Tom Smith, spokesman for the 300-strong British contingent. Kitulagoda's body likely will be flown home next week, he said.
Kitulagoda was killed when a suicide bomber detonated a yellow-and-white taxi next to an open-topped British Land Rover jeep. Four other British soldiers were wounded. The attack came a day after a Canadian soldier was killed in a similar suicide attack.