A US drone fired two missiles into open fields in Pakistan's northwestern tribal belt on Thursday, causing no apparent damage or casualties, local security officials said.

It is highly unusual for no casualties to be reported when unmanned American aircraft target Taliban and Al-Qaeda-linked militants in Pakistan's semi-autonomous tribal belt.

The attack came in the Shin Warsak area of South Waziristan about 20 kilometres (13 miles) from the Afghan border.

The area is a stronghold of Pakistani warlord Mullah Nazir, who officials said was wounded in a suicide attack in the district's main town of Wana earlier on Thursday.

"Two or three militants were walking in a cultivated field. A US drone fired two missiles but no one was hurt," a security official based in the northwestern city of Peshawar told AFP on condition of anonymity.

A second security official in South Waziristan confirmed the drone strike and said that "so far" there was no report of any damage.

The majority of US drone strikes target militants in the neighbouring district of North Waziristan and the last reported attack in Pakistan was on October 24.

The covert strikes are unpopular in Pakistan, where the government in public denounces them as a violation of sovereignty, but US officials believe they are a vital weapon against Islamist militants.