Two US drone strikes on Sunday killed at least six militants in a restive Pakistani tribal region near the Afghan border, security officials said.

That made it three missile attacks in two days in Shawal district, in North Waziristan, after six militants were also killed there on Saturday. North Waziristan is considered a bastion of Taliban and Al-Qaeda-linked insurgents.

"At least two militants were killed and two others wounded when a US drone fired two missiles at the site of this morning's attack, where militants were removing the wreckage of their two destroyed vehicles," a security official told AFP.

Another security official confirmed the latest attack and casualties. A drone strike earlier on Sunday killed four militants, officials said.

The latest strikes came amid reports of a thaw in Pakistan's generally deteriorating ties with the US following a visit to Washington by Pakistan's spymaster, Lieutenant General Zaheer ul-Islam, earlier this month.

Islam's talks with his CIA counterpart were said to have focused on drone strikes.

Attacks by unmanned American aircraft are deeply unpopular in Pakistan, which says they violate its sovereignty and fan anti-US sentiment, but US officials are said to believe the attacks are too important to give up.

The latest attacks were in the same region where a drone strike on June 4 killed 15 militants including senior Al-Qaeda figure Abu Yahya al-Libi.

In protest at the drone attacks, a Taliban and Pakistani warlord, Hafiz Gul Bahadur, has banned vaccinations in North and South Waziristan, putting 240,000 children in the region at risk.

He has condemned the immunisation campaign as a cover for espionage.

In May, a Pakistani doctor was jailed for 33 years after helping the CIA find Osama bin Laden using a hepatitis vaccination programme as cover.

Washington considers Pakistan's semi-autonomous northwestern tribal belt the main hub of Taliban and Al-Qaeda militants plotting attacks on the West and in Afghanistan.

The Al-Qaeda-linked Haqqani network in North Waziristan, blamed for some of the deadliest attacks in Afghanistan, is one of the thorniest issues between Islamabad and Washington.

Washington has long demanded that Pakistan take action against the Haqqanis, whom the United States accused of attacking the US embassy in Kabul last September and acting like a "veritable arm" of Pakistani intelligence.

Pakistan has in turn demanded that Afghan and US forces do more to stop Pakistani Taliban crossing the Afghan border to relaunch attacks on its forces.

There has been a dramatic increase in US drone strikes in Pakistan since May, when a NATO summit in Chicago could not strike a deal to end a six-month blockade on convoys transporting supplies to coalition forces in Afghanistan.

On July 3 however, Islamabad agreed to end the blockade after the United States apologised for the deaths of 24 Pakistani soldiers in botched air strikes last November.