At least four militants have been killed by a US drone strike on Sunday in Pakistan's northwestern tribal belt, 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.

Six US drones flew over the area when one of them fired two missiles at a compound in the Manzarkhel area of the town.

"The drones kept hovering at the compound for a while and then one drone fired two missiles at the time of sunset," a local security official said.

"At least four militants were killed in the strike," said the official.

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

An eyewitness said that the compound caught fire after the strike leaving all the bodies burnt.

"The bodies of the militants were severely burnt and it was hard to identify them," Haji Gul Badin, a local shopkeeper, told AFP.

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 month 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.