At least six militants were killed in two US drone attacks in Pakistan's northwestern tribal region near the Afghan border on Friday, security officials said.

The first drone fired two missiles targeting a suspected militant vehicle in Khaisor town, about 30 kilometres (19 miles) north of Miranshah, the main town in the North Waziristan region, they said.

The attack left three militants dead, one official said. Another security official put the death toll at four.

Two more missiles followed about 15 minutes after the first strike, killing another three militants, local officials said, updating an earlier figure of two.

The second strike came as militants were engaged in rescue work after the first attack, a security official said.

Security officials in Peshawar confirmed the two strikes killed six militants.

The lawless tribal region is known as a hub of Taliban and Al-Qaeda linked militants fighting across the border in Afghanistan.

The United States does not confirm drone attacks, but its military and its Central Intelligence Agency (CIA) operating in Afghanistan are the only forces that deploy them in the region.

The covert US strikes cause anti-American hostility among the Pakistani public, who see foreign military action on Pakistani soil as a violation of national sovereignty.

Missile attacks doubled in the tribal areas last year as the campaign was stepped up, with more than 100 drone strikes killing over 670 people in 2010 compared with 45 strikes that killed 420 in 2009, according to an AFP tally.

The last drone hit on Tuesday, when missiles targeting a compound killed five militants near Wana, the main town in South Waziristan tribal region.

Pakistan tacitly cooperates with the bombing campaign, which US officials say has severely weakened Al-Qaeda's leadership and killed a number of high-value targets, including the former Pakistani Taliban chief Baitullah Mehsud.

Despite pressure from Washington, Pakistan has stalled on launching a ground offensive in North Waziristan, saying its troops are overstretched.