Pakistan has summoned a senior U.S. diplomat to protest against drone strikes on Pakistani soil.

Three separate attacks in the last week have killed at least 16 suspected militants in the North Waziristan tribal agency, a known hub for al-Qaida and Taliban-linked militants.

Pakistan's foreign ministry said Thursday the American diplomat was informed that the strikes were “unlawful, against international law and a violation of Pakistan's sovereignty.” The ministry said such attacks are “unacceptable.” It did not name the senior diplomat.

Pakistan has repeatedly called for an end to U.S. drone strikes, but U.S. officials say they are an important tool in the effort to defeat al-Qaida.

Authorities say militants ambushed a police patrol in the northwestern province of Khyber Paktunkhwa Thursday, killing three policemen. The Taliban claimed responsibility for the attack in the town of Kotki in Hangu district.