Militants armed with rocket-propelled grenades attacked U.S. troops patrolling in the remote northeast of Afghanistan, killing three soldiers before American forces repelled the assault with artillery fire, an official said Saturday.

Three soldiers and one civilian were also wounded in the battle in Waygal district of Nuristan province Friday, said Col. Tom Collins, a chief U.S. spokesman. They were later taken by helicopter to a military hospital for treatment.

"We mourn their loss but their work continues," Collins said, referring to the killed soldiers. "We will honor them by continuing our mission to pursue extremist wherever they are," he added.

He did not say whether the militants suffered any casualties.

In recent weeks U.S. forces have been pushing to their northernmost points along the mountainous Afghan-Pakistan border, including Nuristan, opening military bases in one of the wildest regions in the country.

Their mission is to crush militants loyal to the Hezb-e-Islami militant group of renegade Afghan warlord Gulbuddin Hekmatyar, the toppled Taliban regime and remnants of Usama bin Laden's Al Qaeda network.

Separately, a highway police commander was killed by a blast on his way to work in eastern Lagman province, said Interior Ministry spokesman Yousef Stanezai.

The commander patrolled a major road between Kunar and Nangarhar provinces, which hug the Pakistan border, the spokesman said, offering no further details.

An explosion also occurred outside a NATO base in Kabul early Saturday. No one was injured in the blast, said Maj. Toby Jackman, spokesman for the NATO-led force. It was unclear if it was a bombing or rocket attack by insurgents.

Afghanistan has seen a surge in violence this year, particularly in the south, where rebel supporters of the toppled Taliban regime have stepped up attacks, as Afghan and NATO-led troops try to drive insurgents out of their safe havens.

The fighting has been the bloodiest since the Taliban were ousted in late 2001. In a two-month offensive in the south that ended at the start of August, the coalition claimed to have killed, wounded or captured, some 1,100 militants.

But, Tom Koenigs, the top U.N. official in Afghanistan, told the German news weekly Der Spiegel that the numbers do not reflect success.

"The Taliban fighters' reservoir is practically limitless," Koenigs told the magazine in an interview. "The movement will not be overcome by high casualty figures."