U.S. soldiers battled insurgents who ambushed a military convoy in southern Afghanistan, killing up to 20 militants, the coalition said Monday. A U.S. helicopter also crashed in an accident in southern Afghanistan, killing an American soldier, the U.S. military said. Enemy fire was ruled out as the cause of the downing.

The ambush occurred Sunday in southern Helmand province's Sangin district, which has become a hotbed of militant activity, particularly for the 3,300 British soldiers in the region. Four British troops were killed in attacks in Sangin last week.

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The military said up to 30 extremists, firing guns and mortars, attacked a coalition patrol that had just found a weapons cache in Sangin. About 20 militants were killed, the military said.

Two soldiers were also wounded and airlifted to a medical facility where they were in stable condition, the military said.

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In a separate incident, Afghan police killed seven insurgents that attacked a police checkpoint Sunday night in Nawzad district in southern Helmand province, the Interior Ministry said. No Afghan police suffered casualties, it said.

The battles were part of a massive anti-Taliban offensive in southern Afghanistan involving more than 10,000 Afghan and coalition soldiers. The campaign was launched in earnest last month to halt the deadliest spree of militant violence since the Taliban regime's 2001 ouster.

The commander of British forces in Afghanistan said he has asked for more equipment, and hinted that he may also ask for additional troops.

"In terms of force levels, we constantly keep those under review and I am confident that if I asked for more — and there are some requests which are in staff in process — London would listen to those requests," Brig. Ed Butler said in an interview with British Broadcasting Corp. radio.

"I have put in requests, which are being considered back in London as we speak, to take account of the changing circumstances," he said.

The AH-64 Apache attack helicopter plunged to the ground shortly after taking off from the coalition-run Kandahar Air Field, a military statement said. The cause of the crash was being investigated.

In western Afghanistan, a small explosion Monday wounded eight female students in an English class at Herat University.

Police were investigating who planted the explosive in a trash can, said Nasar Ahmed Piakar, director of the criminal department of Herat police. He said he suspected people who "are against peace and stability. And they don't want education for women."

Taliban extremists oppose women receiving education and have bombed many schools and universities attended by females.

There has been a spike of violence in recent weeks, particularly across southern Afghanistan, where the NATO-led International Security Assistance Force, which includes British troops, is preparing to take over security from the U.S.-led coalition.

Since May, more than 600 Afghans, mostly militants, have been reported killed in Afghanistan, according to an Associated Press tally based on coalition and Afghan figures.

Militants are following the lead of insurgents in Iraq and carrying out suicide attacks and roadside bombings that once were rare in Afghanistan.