Pilotless US drones armed with missiles have stepped up patrols over areas along the border with Afghanistan, hunting for Taliban and Al Qaida militants and fraying nerves below.

Pashtun villagers living on the frontier call them "buzzers", and the aircraft have increasingly taken to the skies, causing sleepless nights and occasionally raining down death.

"We're sick of these drones, they're driving us crazy," said Sher Shah, a government official in the town of Wana in the South Waziristan region.

"They fly so low at night we can't sleep!"

The Predators, capable of carrying two anti-tank Hellfire missiles, can remain aloft for up to 24 hours and beam intelligence live from their hi-tech cameras.

They have struck several times in the northwest this year, killing dozens of suspected militants.

Distinctive buzz

Sometimes villagers can spot the drones - a tiny speck in the sky - and even fire at them with rifles. At other times the drones are too high to see, but you know they're there from the distinctive and incessant buzz given off by their rear-mounted propeller engines.

The buzzing often gets louder at night as the drones patrol at lower altitudes in the darkness, villagers say.

Residents of Bajaur, another militant-plagued area on the Afghan border, to the northeast of Waziristan, said drones flew overhead all night on Thursday.

"The sky is not safe, the earth is not safe, where should we go?" asked Jabbar Shah, a resident of Inayat Kalay village, about 10 km from the border.

"We don't know when they will strike and who they will hit. It's very worrying," he said.

The semi-autonomous tribal belt became a sanctuary for Al Qaida and Taliban militants fleeing from Afghanistan after US-led forces ousted the Taliban in 2001. Al Qaida chief Osama Bin Laden is believed to be hiding somewhere in the mountainous border region.

Taliban militants fighting Western forces in Afghanistan also take sanctuary there and the Pentagon last month said insurgent havens in Pakistan remained the biggest threat to Afghan security.

Islamabad says it is doing all it can to stop attacks into Afghanistan and to rid the region of Al Qaida operatives. Many hundreds of soldiers have been killed battling the militants.

But despite that, analysts say the Predator activity - not officially allowed - is a sign of growing US frustration with a perceived inability to tackle the militants.

Some US politicians, including the Democratic presidential candidate Barack Obama, have even suggested that the United States should attack Al Qaida inside Pakistan without Pakistani approval.

The government, which has been trying to negotiate peace with the militants, has ruled out allowing foreign troops on its soil.

The News newspaper on Friday reported a build-up of US forces on the border in eastern Afghanistan.

But Mehmood Shah, a retired senior security official, said it would be illogical for the United States to open a new front by attacking across the border with troops.

Lurking threat

For the time being, at least, it looks as if the United States will rely on its drones, and people on the border will continue living in fear.

Malik Khardin, a tribal elder in Wana, said he had stopped letting too many cars park outside his house or allowing guests to stay because that might be spotted by the drones.

"We fear we might be hit on suspicion of being Al Qaida," he said.