A US drone fired two missiles into open fields in Pakistan's northwestern tribal belt on Saturday, killing a suspected militant, local security officials said.

The unmanned US drone targetted the suspect who was praying in an open field outside his home 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 on Thursday.

"The targetted militant is suspected to be a key Commander of Al-Qaeda linked Taliban group but his identity is immediately unclear," a security official told AFP.

Another official confirmed the attack, saying that the man killed was a key operative.

"We suspect that the killed militant was a foreigner and a key commander. However, his identity and country of origin was not immediately clear," he said.

The majority of US drone strikes target militants in the neighbouring district of North Waziristan.

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.