At least four people have been killed by a suspected US drone that fired two missiles into a Pakistani tribal area bordering Afghanistan, security officials say.

The missiles hit a house in the village of Sam in South Waziristan, an area known to be a stronghold of Baitullah Mehsud, the head Pakistan's pro-Taliban movement.

"There may be casualties, we are checking," a Pakistani intelligence official, who declined to be identified, said.

A second official said that drones are still flying in the area.

"No one is going close as they fear more missiles could be fired," he said.

According to a local resident in Sam, two big explosions had shaken the village.

US-operated pilotless drones have stepped up strikes in Pakistan since the beginning of September, firing missiles at suspected fighters 11 times and killing dozens of people, Pakistani security officials have said.