"When our forces reached the scene of the explosion, they saw four American soldiers lying on the road covered in blood. They were seriously wounded," said Gen. Salim Khan, deputy police chief of Kandahar. "I do not know if any of them died."
A policeman at the scene, Tor Jan, and several witnesses told The Associated Press that two of the soldiers had died. Khan would not confirm that account, however.
U.S. and Afghan security officials cordoned off the area, warning journalists not to take pictures or ask questions. But the charred remains of one car - presumably the one carrying the bomb - could still be seen.
Kandahar is the main city in southern Afghanistan, and home to a large U.S. military base. It has been the site of frequent attacks - some blamed on Taliban holdouts, other on strife between local factions - usually against convoys making their way on dusty roads.
The attack came a day after Afghan warlord Abdul Rashid Dostum announced his intention to challenge Afghan President Hamid Karzai in the country's elections in October, clouding the U.S.-backed incumbent's chances of a clear victory.
Abdul Rashid Dostum decided to run after securing support across the war-riven country's deep ethnic divides, his spokesman Faizullah Zaki said - and after thousands of supporters feted him at a rally in a northern city.
"He didn't want to depend on his own movement, he wanted more people to support him, and today the people showed that," Zaki said. "He will run for president."
In another development this week, a group of U.S. Marines has pulled out of a Taliban stronghold in southern Afghanistan. It's reported they killed more than 100 enemy fighters during their deployment.
The 2,000 strong force, which lost just one Marine, has withdrawn to an American air base in the southern city of Kandahar and is preparing to leave the country, Col. Frank McKenzie said. The withdrawal already had been announced.
The Marines are returning to American warships "to await further orders." Marines spokesman Maj. Rick Peat says there are no indications that the Marines will be redeployed in Iraq.
The contingent, part of the special-operations capable 22nd Marine Expeditionary Unit, deployed in late March in an attempt to divert militants from attacking preparations for historic elections.
Dostum's Jumbesh militia was part of the Northern Alliance forces which helped the United States drive out the Taliban in late 2001. His supporters have controlled a swath of the north ever since.
Dostum supported Karzai as interim leader, cultivated U.S. and other foreign officials and currently holds the post of presidential security adviser for the north.
But he led opposition to the centralized state enshrined in a new constitution pushed through in January, and was accused by Karzai allies of driving a Kabul-appointed governor from a northeastern province in April.
Jumbesh militias have also feuded relentlessly with those of Dostum's archrival, Tajik general Atta Mohammed, around the key northern city of Mazar-e-Sharif.
Karzai named Mohammed governor of surrounding Balkh province on Tuesday. Zaki denied that the move pushed Dostum into open opposition.
On Thursday, Dostum made a speech before crowds gathered at a hotel in Mazar, saying he had reluctantly accepted their pressure to run.
He urged Afghans in the north and center of the country to register in time for the elections, while insisting that his goal was national unity.
"Everyone has their dignity. I will defend your rights," he said.
He called the new governor his "brother," praised Tajik Defense Minister Mohammed Fahim and said he had spoken also to Karzai, but didn't give details.
Dostum, like another half dozen likely challengers, lacks the national appeal to pose a direct threat to Karzai at the ballot box. A former communist and commander of a feared militia during the country's civil wars, he is widely mistrusted, especially in the Pashtun-dominated south.
But he could win support among fellow ethnic Uzbeks and other minorities and help force a run-off if Karzai fails to secure more than 50 percent in a first round set for Oct. 9.
Zaki said Dostum had already collected the names of 10,000 backers needed to become a candidate and would hand in his nomination to election officials in Kabul by Monday's deadline.