Security officials said a US drone fired two missiles on a militant compound in the North Waziristan region, killing four militants and injuring two others.

The missile strikes have reached record levels in the past month, killing more than 150 people since September 3, amid reported US criticisms of Pakistan's efforts to stamp out the Islamist threat in the border region.

Pakistan's foreign ministry spokesman earlier said there is "no justification" for US drone strikes on its soil that are believed lately to have targeted militants plotting attacks on Europe.

"We believe that (the attacks) are counter-productive and also a violation of our sovereignty," Abdul Basit told reporters, adding, "We hope that the US will revisit its policy."

Basit said the strikes would not help efforts to thwart terrorism, as he said they damaged efforts to win hearts and minds in the region, "which is part and parcel of our strategy against militants and terrorists".

White House spokesman Robert Gibbs did not immediately respond directly to the comments, but described the situation in the tribal areas as "difficult and complex".

The US earlier apologised for a fatal helicopter strike in Pakistan, as it presses its key anti-terror ally to reopen a key border crossing to Afghan-bound NATO supply trucks that have come under repeated Taliban attack.

But the diplomatic overture reaped no immediate reward and Basit said there was no set date for reopening the main border post at the Khyber Pass.

The US has launched 27 drone attacks in a new wave since September 3, while more than 1,100 people have died in 143 strikes since the campaign began in August 2008.

Officials in Washington say previous drone strikes have killed a number of high-value targets including former Pakistani Taliban chief Baitullah Mehsud.

Pakistan's ambassador in Washington, Hussein Haqqani, told the BBC that the increase in strikes came after intelligence agencies uncovered a plot to attack multiple targets in Europe.

The Al-Qaeda plot reportedly targets Britain, France and Germany with a wave of commando-style attacks on key landmarks including Paris's Eiffel Tower and Notre Dame Cathedral and Berlin's Brandenburg Gate.

Pakistan was hit by a double bombing in Karachi Thursday, when at least eight people were killed at a Sufi shrine in what police said was a suspected suicide attack.

Meanwhile, US ambassador Anne Patterson and the top US military official Admiral Mike Mullen have tried to publicly repair relations damaged by the cross-border NATO chopper strike last Thursday.

Two Pakistani soldiers mistaken for militants were killed in the attack and Pakistan shut the main land route at Torkham following the intrusion into its territory.

"We extend our deepest apology to Pakistan and the families of the Frontier Scouts who were killed and injured," Patterson said in a statement released late Wednesday.

"Pakistan's brave security forces are our allies in a war that threatens both Pakistan and the US."

In a letter to Pakistan's army chief General Ashfaq Kayani, Mullen on Thursday said the US military took the incident "very seriously" and offered his condolences to the families of the dead.

Mullen said senior commanders would review an investigation into the incident thoroughly in the hope of "avoiding recurrence of a tragedy like this."

About 6,500 oil tankers and supply vehicles have been stranded in Pakistan for more than a week, waiting for the route to reopen to supplies heading for the 152,000 US-led troops fighting a nine-year war in Afghanistan.

About 120 NATO vehicles have been destroyed in gun and arson attacks over the past week since the border crossing was shut, as Taliban militants step up efforts to disrupt the supply route and avenge the US drone strikes.

In the latest attack 54 NATO oil tankers were torched in a militant attack on a convoy parked in Nowshera in northwestern Pakistan, police said Thursday.

Delicate relations between the two countries could be further strained by a White House report to Congress Wednesday which warned Pakistani forces were avoiding "direct conflict" with the Afghan Taliban and Al-Qaeda in the northwest tribal zone.

The report said the Pakistani military had continued operations against insurgents in lawless South Waziristan, but added that soldiers stayed close to roads and that operations were progressing "slowly".

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