Three U.S. soldiers have been killed in heavy fighting with Taliban guerrillas in northeastern Afghanistan, the U.S. military said on Saturday.

Three other U.S. soldiers and an Afghan colleague were wounded in the battle on Friday close to the border with Pakistan, when rebels attacked their patrol with rocket-propelled grenades and small arms.

U.S. forces called in artillery support to repel the attack.

"These soldiers were fighting extremists who are against progress and good governance in Afghanistan," the spokesman for U.S.-led coalition forces, Colonel Tom Collins, told reporters.

"We mourn their loss, but their work continues."

Taliban fighters are active in Nuristan, a province of heavily forested mountains. Less than a week ago 12 guerrillas were killed during an attack on a U.S. base. U.S. forces are trying to set up permanent bases in the area.

In the volatile south, a NATO soldier was killed when a suicide car bomber attacked a convoy on the main highway between Kandahar province and Pakistan on Friday.

Another NATO soldier was seriously wounded in fighting with suspected Taliban fighters in neighboring Helmand province.

Violence in Afghanistan is at its worst since U.S.-led forces toppled the Taliban government, allied to al Qaeda, in 2001.

The bloodshed is particularly bad in the Taliban’s southern heartland, where it has regrouped and is staging a series of coordinated attacks and bombings against Afghan and NATO troops.

The fighting increased as NATO built up its presence ahead of taking over the region from U.S. forces on July 31.

Copyright © 2006 Reuters