— Afghan and U.S. troops foiled a complex attack by Taliban insurgents against a NATO-led base Saturday in Panjshir Valley in eastern Afghanistan, one of the few secure regions that so far has remained free of militants’ strikes.

Armed with hand grenades, anti-tank rockets and assault rifles, a group of five attackers including at least two suicide bombers launched the pre-dawn raid on the base, perched in the foot of a hill near the only main road in the twisting valley.

There were no casualties among U.S. military troops and American civil personnel running the base in Rakha district of Panjshir province, but two Afghan drivers supplying fuel for the base were killed in the clash, provincial authorities and a spokesman for the NATO-led troops said.

“There was a complex attack attempted, but it was repelled,” NATO spokeswoman Capt. Ebony Calhoun told the Associated Press. She said the guards’ wounds were not life-threatening but they had been evacuated to a larger U.S. base for treatment.

The AP reported that the Taliban claimed responsibility for the attack in a text message.

Two Afghans guarding the base were wounded and its main entrance was damaged by a massive car bomb used in the initial round of the attack by a suicide bomber.

Four other assailants, dressed as athletes, hurled hand grenades and rockets at a tower of the base, sparking a clash that lasted for nearly half an hour. NATO-led forces had to use air power to repel the attack, an official said.

While insurgents in the past have unleashed a number of similar attacks against foreign troops and Afghan government in various parts of the country, Saturday’s raid was rare in its kind in Panjshir, the only few secure areas of Afghanistan.

The valley served as the main “unconquerable” stronghold of the anti-Taliban alliance that helped the U.S. troops drive out the Taliban from power in 2001.

The attack came as a surprise to Afghans and foreign forces with Panjshir’s governor saying an investigation was launched to determine how the attackers were able to penetrate into the valley.

“A team has come from Kabul for assessing this incident. We are trying to establish how the attackers came here and were able to conduct such a raid,” Gov. Keramuddin Keram said by phone.

Though a former stronghold of the anti-Taliban Islamists, Panjshir is largely conservative and some of its people have regularly voiced anger about the 10-year-long presence of foreign troops and Western policies in Afghanistan.