A US drone strike killed four militants Monday and destroyed a rebel compound in Pakistan's lawless tribal badlands along the Afghan border, local security officials said.

Pakistani officials say unmanned US aircraft have significantly stepped up attacks on Taliban and Al-Qaeda-linked operatives in the semi-autonomous region this month, clocking up at least 19 attacks in 24 days.

Monday's strike took place in Khushali Toorikhel village southeast of Miranshah, the main town in North Waziristan, the tribal district that has been the focus of the covert drone campaign as a Taliban and Al-Qaeda stronghold.

"We can now confirm that four militants were killed when a US drone fired two missiles on a militant compound," a senior Pakistani security official told AFP on condition of anonymity.

Another security official confirmed the attack and the death toll.

"We don't yet know the identities of those killed," the official said.

Pakistani security officials also said US drones carried out missile strikes in North Waziristan on Saturday and Sunday, killing at least 11 militants.

Militants fighting against 150,000 US and NATO troops in Afghanistan are believed to be holed up in Pakistan's northwestern Federally Administered Tribal Areas, as are operatives at war with Pakistani security forces.

Washington has branded the rugged tribal area, which lies outside Pakistani government control, a global headquarters of Al-Qaeda and the most dangerous place on Earth.

The US military does not as a rule confirm drone attacks, but its armed forces and the Central Intelligence Agency operating in Afghanistan are the only forces that deploy pilotless drones in the region.

More than 1,100 people have been killed in over 130 drone strikes in Pakistan since August 2008, including a number of senior militants.

Recent missile attacks have largely targeted militants linked with the Haqqani network, which is based in North Waziristan.

Officials in Washington say the strikes have killed a number of high-value targets including Pakistani Taliban chief Baitullah Mehsud. However, the attacks fuel anti-American sentiment in the conservative Muslim country.

Under US pressure to crack down on Islamist havens along the Afghan border, Pakistan has in the past year stepped up military operations against largely homegrown militants in the area.

Last year Pakistan launched its most ambitious military offensive yet against Taliban militants in South Waziristan, expanding the campaign to many of the other seven semi-autonomous tribal districts along the border.

Pakistani commanders have not ruled out an offensive in North Waziristan, but argue that gains in South Waziristan and the northwestern district of Swat need to be consolidated to prevent their troops from being stretched too thin.

Pakistan protests over cross-border NATO air strikes
Islamabad (AFP) Sept 28, 2010 - Pakistan on Monday denounced cross-border air strikes by NATO helicopters pursuing militants as a violation of its sovereignty as the alliance said its forces had the right to defend themselves.

The NATO-led International Security Assistance Force (ISAF), which is fighting a Taliban-led insurgency in neighbouring Afghanistan, said the attack Friday left more than 30 rebels dead.

Pakistan said the helicopters intruded into its territory twice from the eastern Afghan province of Khost as they chased the militants.

"These incidents are a clear violation and breach of the UN mandate under which ISAF operates," foreign office spokesman Abdul Basit said in a statement.

ISAF said the choppers "engaged a significant number of insurgents" after a remote Afghan forces outpost in Khost province came under attack.

"An air weapons team in the area observed the enemy fire, and following International Security Assistance Force rules of engagement, crossed into the area of enemy fire," it said in a statement.

ISAF aircraft then fired on the militants killing more than 30, the statement said, adding that two helicopters returned to the border area on Saturday and killed several more insurgents.

"Initial reports indicate no civilians were injured or killed during either operation," the ISAF statement said. The force later added that no ground troops had crossed the border during the operation.

"The helicopters briefly crossed the border to engage the threat to the outpost," an ISAF spokesman told AFP.

An angry Pakistan said ISAF's mandate "finishes" at the Afghan border.

"There are no agreed 'hot pursuit' rules. Any impression to the contrary is not factually correct. Such violations are unacceptable," the foreign office statement added.

It said ISAF had been asked not to participate in any military action that violates its UN mandate and infringes upon Pakistan's sovereignty.

"In the absence of immediate corrective measures, Pakistan will be constrained to consider response options," it warned.

Responding to Islamabad's protest, NATO said its forces have a right to self-defence.

"ISAF forces must and will retain the authority, within their mandate, to defend themselves in carrying out their mission," a NATO official in Brussels told AFP.

The official expressed "regret that ISAF was unable to establish contacts with the Pakistani military, despite attempting to do so, as the incident was taking place."

He said ISAF would continue to work closely with its Pakistani partners, "both to stop cross-border attacks from Pakistan into Afghanistan and also to enhance coordination and communication."

The incident is likely to further complicate Washington's ties with Islamabad as well as Afghanistan's difficult relations with Pakistan.

Islambabad last week rejected revelations by famed Watergate journalist Bob Woodward that the CIA runs a secret Afghan paramilitary force that hunts down Al-Qaeda and Taliban militants in covert operations in Pakistan.

US President Barack Obama has sought to pile pressure on militant havens in Pakistan through a stepped up bombing campaign using unmanned aircraft as well as US special forces' operations in Afghan territory.

The US military's presence in Afghanistan and its covert drone strikes in the border tribal belt are subject to sharp criticism and suspicion in Pakistan.

The rare NATO cross-border attacks came amid a surge in drone strikes in the tribal belt, which is considered a safe haven for Taliban and Al-Qaeda-linked operatives and described by Washington as the most dangerous place on Earth.

Four militants were killed in a missile strike on Monday which was 19th attack in 24 days.

Under US pressure, Pakistan has stepped up military operations against largely homegrown militants.

Last year Pakistan launched its most ambitious military offensive yet against Taliban militants in South Waziristan, expanding the campaign to many of the other seven semi-autonomous tribal districts along the border.