At least four people were killed late on Friday night when a U.S. drone launched an attack in North Waziristan tribal agency in northwest Pakistan, local sources said.

Three others were also injured after the pilotless aircraft fired three missiles into a village in Mir Ali, a main town in North Waziristan near the Afghan border.

Details about the killed are not immediately known. Most of the killed are believed to be militants hiding in the area, but the U. S. drones do sometimes mistakenly kill innocent civilians.

It was the second strike of its kind in a day. In an afternoon attack, at least four people were killed, including Taliban commander Qari Hussain.

Hussain is believed to be a mastermind of many suicide bomb attacks across Pakistan, sources said.

Since a suicide attack killed seven CIA employees in Afghanistan in December, covert U.S. drone attacks have tremendously increased in the volatile Waziristan tribal region.

Waziristan is the home of Taliban leaders Hakimullah Mehsood, Hafiz Gul Bahadur and Mulla Nazir, who are fighting against U.S. and NATO forces in Afghanistan.

Al-Qaeda-linked Haqqani network, whose network is fighting against U.S. and local forces in neighboring Afghanistan, is also active in the region. The network is run by Sirajuddin Haqqani, the son of former Taliban commander Jalaluddin Haqqani.

Pakistan is under pressure to launch operation against Haqqani network. A number of high-profile militant leaders, including Pakistani Taliban chief Baitullah Mehsud were killed in the drone attacks.

Pakistan publicly criticizes drone attacks, saying they violate its sovereignty and fuel more anti-Americanism among the people, but observers widely believe that Pakistan shares intelligence with the U.S. on such strikes.