Pakistan summoned a US diplomat on Thursday to protest over drone strikes in its northwestern tribal area, which it called "unlawful", the foreign ministry said in a statement.

Missile attacks from unmanned US aircraft in the restive North Waziristan tribal district have killed at least 15 people since the start of the Muslim festival of Eid on Saturday.

"A senior US diplomat was called to the Ministry of Foreign Affairs and informed that the drone strikes were unlawful, against international law and a violation of Pakistan's sovereignty," the ministry statement said.

Pakistan has repeatedly criticised the drone strikes in public, calling them counter-productive.

In June Richard Hoagland, the US charge d'affaires -- currently the most senior US diplomat stationed in Pakistan -- was summoned to the foreign ministry to hear a protest about drones.

This week's strikes came amid reports of a thaw in Islamabad's difficult ties with Washington following a visit to Washington by Pakistan's spymaster, Lieutenant General Zaheer ul-Islam, earlier this month.

Islam's talks with his CIA counterpart were said to have focused on drone strikes.

Attacks by unmanned American aircraft are deeply unpopular in Pakistan, which says they violate its sovereignty and fan anti-US sentiment, but US officials are said to believe the attacks are too important to give up.