Insurgents killed four U.S. soldiers and wounded six others in separate clashes in Afghanistan on Saturday as the war-battered nation celebrated its independence day amid an upsurge in violence that threatens its democratic rebirth.

Two Afghan security forces were also killed in the worst bout of fighting since the fall of the Taliban regime after the U.S.-led invasion of Afghanistan in late 2001.

Three U.S. soldiers were killed and three others wounded during a fight with Taliban militants in Pech district in eastern Kunar province, said Col. Tom Collins, the U.S. military spokesman.

Their combat patrol struck an improvised explosive device before they engaged "a group of Taliban extremists," a U.S. military statement said. "Enemy forces were attacked with small arms and artillery fire," it said, adding that insurgent causalities "have yet to be reported."

CountryWatch: Afghanistan

American troops in the area are hunting for Taliban fighters and extremists close to Usama bin Laden's Al Qaeda network in remote mountains hugging the Pakistani border.

Also Saturday, a U.S. soldier was killed and three others wounded in a four-hour clash with more than 100 insurgents in southern Uruzgan province, officials said. One Afghan soldier also died.

The troops used artillery and air support to repel the insurgent attack, a NATO statement said.

The slain American was part of a U.S. team training the fledgling Afghan National Army, Collins said. Currently about 22,000 U.S. forces are in Afghanistan along with 20,000 NATO-led troops.

As of Friday at least 266 members of the U.S. military have died in Afghanistan, Pakistan and Uzbekistan as a result of the U.S. invasion of Afghanistan in late 2001, according to the Defense Department. Of those, the military reports 165 were killed by hostile action.

Saturday's spate of violence occurred as insurgency-wracked Afghanistan celebrated its independence from British rule. The holiday marks liberation from Britain in 1919, following the third Anglo-Afghan war.

President Hamid Karzai told thousands in Kabul's richly decorated stadium that education was the key to protecting Afghanistan's independence amid efforts by militants to undermine his authority by also burning schools.

"Our history proves our bravery," he said in the stadium adorned with pictures of former and current leaders. "The only thing we need to keep our independence is education."

Militants have targeted schools, burning 144 to the ground over the past year and forcing another 200 to close following threats against teachers and students, according to Afghan officials. More than 200,000 Afghan children have consequently been unable to continue their education.

The Taliban claims educating girls is against Islam and oppose government-funded schools for boys because they teach secular subjects besides religion.