An American soldier was killed Monday and two were wounded when a bomb exploded near their patrol in southern Afghanistan, while warplanes pounded militants holed up in mountain caves nearby.

Taliban militants, meanwhile, killed two policemen south of the capital and threw a grenade at a relief group in the northwest — signs that violence is spreading ahead of historic national elections in September.

The attack on the U.S. soldiers occurred when a bomb hit their Humvee near Deh Rawood, about 250 miles southwest of the capital in the violent southern province of Uruzgan (search), spokeswoman Lt. Col. Michele DeWerth said.

She said all three were flown to Kandahar Air Field Hospital (search), where one soldier died. She would not provide further details.

Uruzgan Gov. Jan Mohammed Khan said U.S. forces had surrounded the area of the explosion and were not allowing Afghan troops in.

The death brought the total number of American service personnel who have died in and around Afghanistan since the start of the U.S. war on terrorism to at least 91, including 54 killed in action.

A mine killed four American special forces on May 29 when they were traveling in a Humvee in neighboring Zabul province, one of the worst American combat losses since the war that drove the Taliban from power in late 2001.

More than 400 people have died in violence across Afghanistan this year, most in the south and east where U.S. and Afghan forces have killed dozens of suspected militants in recent weeks.

The U.S. military has assembled 20,000 troops, its largest-ever force in Afghanistan, in an attempt to prevent rebels from derailing the country's first post-Taliban election.

But there are signs the insurgency is expanding.

The warplanes struck early Sunday near Tirin Kot, the capital of Uruzgan, after U.S. troops had exchanged fire with dozens of militants who sought refuge in the caves, U.S. military spokesman Lt. Col. Tucker Mansager said.

The Americans called in "air support that dealt with those caves," Mansager said.

He said no U.S. soldiers were hurt in the battle and had no information on casualties among the militants.

The policemen died when Taliban attacked the government office in Kharwar, a remote district of Logar province 50 miles south of Kabul, said Gen. Atiqullah Ludin, a local military commander.

Ludin said about two dozen assailants rode into town in four-wheel-drive pickup trucks and opened fire with guns and rocket-propelled grenades, setting fire to one office.

That attack came less than a week after five workers for the medical relief agency Medecins Sans Frontieres (search), including three foreigners, died in northwestern Badghis province.

Aid groups worry that relatively secure provinces such as Badghis and Logar will join the south and east in being too dangerous for badly needed reconstruction work.

The Italian relief group Alisei on Monday reported a fresh incident in Badghis, a grenade attack on its compound in the provincial capital, Qalay-e Naw.

The grenade was tossed over an outer wall late Sunday, damaging a vehicle and a water tank, but injuring no one, Alisei spokesman Fabio Amici said.

Provincial police chief Amir Shah Naibzada said the Taliban were probably to blame, but also alluded to factional tensions in the region.

Huub Verhagen, a senior MSF official in Kabul, said its operations would remain suspended nationwide until at least Sunday while it analyzed how its staff died.

Election workers have also been targeted, most recently on Sunday, when militants ambushed a U.N. convoy in southeastern Paktia province. Guards and the assailants fought a pitched battle, but no one was hurt.

Karzai insisted last week that the vote should go ahead, though the United Nations, which is scrambling to register millions of voters around the country, says security must improve.

Karzai left Monday for a trip to the United States, where he was expected to discuss security and elections in Afghanistan with President Bush and other American officials.