U.S. warplanes pounded dozens of insurgents hiding in caves in southern Afghanistan (search), the military said Monday, after a gunbattle between the militants and U.S. troops. An American soldier was killed when a bomb hit his patrol in a separate incident.

Meanwhile, Taliban (search) militants killed two policemen south of the capital and threw a grenade at a relief group in the northwest, officials said, fresh signs that violence is spreading ahead of crucial national elections.

The soldier was killed and two others wounded by a bomb during a patrol in the violent southern province of Urzugan, the U.S. military said.

The explosion hit the soldiers' Humvee near Deh Rawood, 250 miles southwest of the capital, Kabul, spokeswoman Lt. Col. Michele DeWerth said.

The planes struck early Sunday near Tirin Kot, a town 250 miles southwest of Kabul where U.S. Marines recently set up a base, military spokesman Lt. Col. Tucker Mansager said.

The militants sought refuge in the caves, and coalition forces called in "air support that dealt with those caves," Mansager said.

He said no U.S. soldiers were hurt and had no information on any casualties among the militants, who he said numbered "probably in the tens and twenties."

More than 400 people have died in violence across Afghanistan this year, most in the south and east where U.S. forces and Taliban militants have clashed repeatedly in recent weeks.

The U.S. military has assembled 20,000 troops, its largest-ever force in Afghanistan, in an attempt to keep militants on the defensive in the run-up to the vote.

But there are signs that the insurgency is expanding — particularly new attacks in previously secure provinces like Badghis and Logar.

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

Two dozen assailants in four-wheel-drive pickup trucks opened fire with guns and rocket-propelled grenades, setting fire to one office, he said.

Ludin said two police officers were killed and another injured before the Taliban withdrew into the mountains. An Interior Ministry spokesman in Kabul said only one policeman had died.

In northwestern Badghis province, a grenade was tossed over the wall of the compound housing the Italian relief group Alisei in the provincial capital, Qalay-e Naw.

The grenade was thrown 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.

Last week, gunmen killed five medical relief workers, including three foreigners, in Badghis in an attack claimed by the Taliban.

The violence is also a threat to plans to hold elections in September.

Militants ambushed a U.N. convoy of election workers in southeastern Paktia province on Sunday. Guards and the assailants fought a pitched battle, but no one was hurt.

Afghan President Hamid Karzai (search) 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.

In another incident further south, militants opened fire on U.S.-led forces Friday with small arms and machine-guns north of Spin Boldak, a town on the Pakistani border, Mansager said.

There were no reports of injuries, but five militants were detained.