Afghan officials earlier said 13 Taliban were killed and eight wounded and captured in the fighting that began late Wednesday in Kandahar province, the former ruling militia's heartland, and ended Thursday.
Master Sgt. Cindy Beam, an American military spokeswoman, said three U.S. Marines were slightly wounded.
A 2,000-strong Marine force was recently deployed in southern Afghanistan to step up the hunt for al Qaeda and Taliban holdouts. Some 20,000 U.S. forces — up from 11,000 at the start of the year — are now in the country, trying to secure volatile areas in the south and east ahead of crucial general elections due in September.
Khalid Pashtun, spokesman for the Kandahar provincial government, told The Associated Press on Thursday that some 300 Afghan troops and a smaller number of Americans had skirmished with gunmen in Kandahar's Miana Shien district, some 150 miles southwest of the capital, Kabul.
Beam confirmed the joint operation and that U.S. warplanes joined the fray, but gave no other details.
"We have also confirmed 17 enemy KIAs (killed in action)," Beam said in an e-mail.
The clash appeared to be the most deadly since fierce fighting in late August and early September in the mountains of neighboring Zabul province, in which well over 100 Taliban are believed to have died.
Militants have since changed tactics, operating in smaller groups that have killed scores of aid workers and government officials as well as Afghan and foreign troops, casting serious doubt over U.S. promises they can crush the insurgency this year.
Four American special forces were killed last Saturday in the same province when a mine exploded under their Humvee.
At least 89 American service personnel have died in and around Afghanistan since the start of the U.S. war on terrorism following the Sept. 11 attacks, including 55 killed in action.
In a worrying development that could signal the insurgents are widening their field of operations, five relief workers, including three foreigners, were gunned down in northwestern Afghanistan on Wednesday.
A purported Taliban spokesman claimed responsibility for the assault, the deadliest on international aid workers since the ouster of the fundamentalist regime by U.S-backed forces in late 2001.
At least 33 aid workers, mostly Afghans, have been killed since March 2003 — but mostly in the south and east where the rebels are most active.
Medecins Sans Frontieres, which has nearly 1,500 staff in Afghanistan, immediately suspended its operations across the country. Authorities in Badghis province said the suspension had an immediate effect, closing several MSF-run clinics and leaving hundreds of people in the impoverished region without medicine.
"It's a very poor and remote province," said Amir Shah Naibzada, the provincial chief of police. "This has had a big impact on health work."
He blamed the attack on the Taliban, claiming that there were no other armed factions in the province who could be responsible.
Poor security is also hurting efforts to register the estimated 10 million voters for the September elections. The process is lagging in volatile southern and eastern provinces, and has been suspended in Badghis after Wednesday's attack.
President Hamid Karzai on Thursday played down security fears, and insisted the election would go ahead, but conceded the number of registered voters could be considerably less than originally hoped.
"We hope to reach at least 6-7 million people. If it is evenly distributed around the country, it will be the best thing," he told reporters, saying more than three million Afghans have signed up so far.
In a press conference on Tuesday, President Bush said, "the reports from Afghanistan, at least the ones I get, are very encouraging."
"You know, we've got people who have been there last year and have been back this year report a different attitude. And they report people have got a sparkle in their eye," the president said.
But a General Accounting Office report released Wednesday — while crediting U.S. aid for helping to avert a humanitarian crisis — found that in the last two fiscal years "the postconflict environment in Afghanistan threatened progress toward U.S. policy goals, and poor security, increasing opium cultivation, and inadequate resources impeded U.S. reconstruction efforts."