Pakistan says it has lodged a formal protest with the U.S. ambassador in Islamabad, following air strikes that killed 18 people in a tribal area on the Afghan border, considered a haven for militants and Afghan insurgents. News reports, citing unnamed intelligence sources, say al-Qaida's second in command, Ayman al-Zawahiri, was the intended target.

Pakistani tribal villagers view damage caused by airstrikes in village of Damadola, near the Afghan border

Friday's missile attack reportedly destroyed a suspected al-Qaida safe house in the Bajur tribal area bordering Afghanistan.

A Pakistan Foreign Ministry statement says a preliminary investigation suggests foreigners were present in the area, and that the strikes were launched from across the border.

The statement condemned the civilian deaths, saying a formal protest has been lodged with U.S. diplomats in Islamabad.

U.S.-led forces in Afghanistan have come under frequent attacks from Taleban insurgents in the border area.

Pakistani officials say Friday's attack is still under investigation, and there is no confirmation of U.S. involvement, nor whether al-Zawahiri was the intended target.

The Egyptian-born militant has been on the run since late 2001, after U.S.-led forces invaded Afghanistan.

Sheikh Rashid Ahmed

Pakistan's information minister, Sheikh Rashid Ahmed, says there is no evidence suggesting al-Zawahiri was present when the attack took place.

"We deeply regret that civilian lives have been lost,"  he said. "We want to assure the people we will not allow such an incident to reoccur."

Thousands of tribesmen from the region staged anti-American protests Saturday.

Pakistan's tribal belt, which runs parallel to the Afghan border, is considered a relative safe haven for suspected militants. Both al-Zawahiri and al-Qaida leader Osama bin Laden are thought to be hiding somewhere in the border area.

Pakistan, a key U.S. ally in the war on terror, has deployed some 70,000 troops to the region to secure the border and flush out suspected militants.