US drones fired two missiles at a vehicle in Pakistan's northwestern tribal belt Thursday, killing four militants, security officials said.

The attack took place in Datta Khel town, 35 kilometres (22 miles) west of Miranshah, the main town in the lawless North Waziristan region, which borders Afghanistan. The area is a stronghold of Taliban and Al-Qaeda-linked militants.

"Four drones came to the area around midnight and fired two missiles targeting a militant vehicle," a local security official said.

"At least four militants were killed, their vehicle was blown up," he said.

Another security official in the northwestern city of Peshawar, which lies on the edge of the tribal region, confirmed the attack and the casualties.

The covert strikes are publicly criticised by the Pakistani government as a violation of sovereignty but American officials believe they are a vital weapon in the war against Islamist militants.

A UN envoy last week said US drone attacks violate Pakistan's sovereignty.

UN special rapporteur on counter-terrorism and human rights Ben Emmerson visited Islamabad as part of an investigation into civilian casualties caused by drone strikes.

According to Britain's Bureau of Investigative Journalism, CIA drone attacks in Pakistan have killed up to 3,577 people since 2004, up to 884 of them civilians.