Pakistan has appointed a new head of its powerful military spy agency.

Prime Minister Yousuf Raza Gilani on Friday named Lieutenant-General Zaheer-ul-Islam as the new director-general of the Inter-Services Intelligence agency, or ISI.

Islam replaces Lieutenant-General Ahmad Shuja Pasha, who is retiring as chief of the ISI. Pasha was appointed to the post in 2008. His tenure saw some of the tensest moments in U.S.-Pakistan relations, including the U.S. raid that killed Osama bin Laden in the garrison Pakistani town of Abbottabad.

U.S. and Pakistani intelligence officials have cooperated in the fight against al-Qaida. But there are lingering doubts in Washington about the ISI's loyalty.

On Friday, Pakistani officials said a U.S. drone strike killed at least 12 militants in the country's South Waziristan tribal region, near the Afghan border.

In neighboring North Waziristan, authorities said militants ambushed a military patrol, killing seven soldiers. Pakistani forces returned fire, killing eight militants.

Elsewhere in the tribal region, officials say a militant shot and killed the head of a local pro-government militia in Orakzai.

Pakistan's tribal regions are a hub for Taliban and al-Qaida-linked militants.