US missiles killed eight militants in separate strikes Wednesday on Pakistan's lawless tribal district, an insurgent stronghold along the Afghan border, security officials said.

Washington has called Pakistan's semi-autonomous northwest tribal region the most dangerous place on Earth and the global headquarters of Al-Qaeda, where Taliban and other Al-Qaeda-linked networks have established bases.

In the first attack a drone fired two missiles into a car on a roadside near Wana, capital of South Waziristan, a Pakistani security official told AFP on condition of anonymity as he was not authorised to release the information.

He said the car was destroyed and that witnesses said all four people in the vehicle had been killed.

The attack took place close to where Pakistani officials said a similar strike likely killed Al-Qaeda commander Ilyas Kashmiri earlier this month, although there has still been no official confirmation of his death.

Officials said the identities of Wednesday's dead were not clear but the Kari Kot area, where the drone targeted, is dominated by militants loyal to Pakistani warlord Maulvi Nazir who fight against US troops in Afghanistan.

The second missile strike also targeted a vehicle and took place in North Waziristan's Tapi village, 10 kilometers (six miles) east of the main town Miranshah.

"US drones fired four missiles into a militant vehicle killing at least four militants," a security official told AFP.

Witnesses said fire engulfed the vehicle burning the occupants instantly.

Fifteen US drone strikes have now been reported in Pakistan's tribal belt since US commandos found and killed Al-Qaeda chief Osama bin Laden in a unilateral airborne raid on the Pakistani garrison city of Abbottabad on May 2.

A senior US anti-terrorism official said that bin Laden's death had had a "greater" impact than expected on efforts to dismantle Al-Qaeda, believing that he was more involved in running the network than once thought.

State Department coordinator for counterterrorism Daniel Benjamin said Al-Qaeda operatives are now on the move "because of a certain amount of disorientation and uncertainty about their own security".

Relations between Pakistan and the United States, wary at the best of times, deteriorated sharply over the raid, which humiliated the Pakistani military and invited allegations of incompetence and complicity.

The New York Times reported Wednesday that Pakistan's Inter-Services Intelligence agency has arrested five Pakistani informants who assisted the CIA ahead of the raid that killed bin Laden.

The men reportedly include an army major said to have copied the licence plate of cars that drove up to bin Laden's compound in Abbottabad, two hours from the capital Islamabad.

Pakistan's military denied that any army officer had been detained, dismissing the story as "false and totally baseless".

The paper said the CIA deputy director rated Pakistan's counterterrorism cooperation with the United States as three out of 10.

The Obama administration acknowledged that relations with Islamabad were "complicated", but said ties remained strong.

"The cooperation that we do get is vital and essential to our war against terrorists and terrorism," White House spokesman Jay Carney said.

"Our relationship with Pakistan is extremely important. It is also complicated."

The United States does not officially confirm Predator drone attacks, but its military and the CIA operating in Afghanistan are the only forces that deploy the armed, unmanned aircraft in the region.

The strikes are hugely unpopular among a Pakistani public deeply opposed to the government's alliance with Washington, and are seen to add pressure on Pakistan to open a new front against militants in North Waziristan.