A US drone strike early Wednesday killed four militants in the northwestern tribal area of Pakistan, local officials said.

Four unmanned aircrafts fired as many missiles on a compound in the main market area of Miranshah, the main town of North Waziristan, which is known as a stronghold of Taliban and Al-Qaeda-linked militants.

It was the second US drone attack inside Pakistan since Nawaz Sharif was sworn in as prime minister in June, calling for an end to such strikes and complaining that they violated the country's sovereignty.

The previous attack, in which seven militants were killed, came on June 7 just two days after Sharif, of the centre-right Pakistan Muslim League (PML-N) party, was sworn in.

Drones were seen flying over Miranshah's main bazar for hours before launching a strike at the militants' compound in the early hours of Wednesday.

"Four drone aircrafts were flying over the area at the time of the attack, while two of them fired four missiles on a compound, killing four militants and wounding two others," a local security official told AFP.

Another official in Peshawar confirmed the attack.

"The attack came when the militants were sleeping in the targeted compound. The death toll from the attack may rise," the official told AFP on condition of anonymity because he was not authorised to speak to media.

An AFP reporter saw the drones flying over the area even after the attack.

Drone missile strikes are very unpopular in Pakistan, but Washington views them as a vital tool in the fight against Taliban and Al-Qaeda militants holed up in the lawless tribal areas along the border with Afghanistan.

Ties with Washington will be a key part of Sharif's tenure, particularly as NATO withdraws the bulk of its forces from Afghanistan by the end of next year after more than 12 years of war.