Pakistan on Monday denied provoking NATO air strikes that left 24 Pakistani soldiers dead, raising tensions over the lethal cross-border attack that has plunged US-Pakistani relations to a new low.

NATO and the United States have sought to limit the fallout from Saturday's attack, which has seen Pakistan close a vital lifeline to the 140,000 foreign troops serving in Afghanistan and order a review of its US alliance.

Washington has backed a full inquiry and expressed condolences. NATO chief Anders Fogh Rasmussen has spoken of regret over the "tragic, unintended" killings, but stopped short Sunday of issuing a full apology.

The crisis erupted months after the fraught US-Pakistan alliance was plunged to its lowest point in years by the killing in May of Al-Qaeda leader Osama bin Laden north of Islamabad by US special forces.

But few questions have been answered about what exactly happened at the dead of night in some of the most hostile terrain on Earth, following reports that Pakistani soldiers opened fire first on US and Afghan forces.

The Wall Street Journal, following a similar report by Britain's Guardian newspaper, cited three Afghan officials and one Western official as saying the air raid was called in to shield allied forces targeting Taliban fighters.

NATO and Afghan forces "were fired on from a Pakistani army base", the unnamed Western official told the Journal. "It was a defensive action."

An Afghan official said the government in Kabul believes the fire came from the Pakistani military base -- and not from insurgents in the area.

An Afghan border police commander, speaking on condition of anonymity because officials have been told not to speak to media before an investigation is completed, said NATO troops hardly ever open fire unless they are attacked.

"To me it's almost clear that they (ISAF) came under fire from that area. Without that they would have not returned fire," he told AFP.

He said the area is very rugged, mountainous and heavily wooded. He said Taliban, Afghan security forces as well Pakistani security forces have posts very close to each other due to the rugged terrain.

Pakistan insists the attack was "unprovoked". There has been no official US response to the report.

"This is not true. They are making up excuses. And by the way, what are their losses, casualties?" Major General Athar Abbas, Pakistan's chief military spokesman, wrote to AFP in a text message.

British newspaper The Daily Telegraph on Monday quoted wounded survivors of the raid, who insisted they were victims of an unprovoked attack.

Amirzeb Khan, 23, was quoted as saying that the area around the checkpoints, about three kilometres (two miles) from the border, had been cleared of militants and the night had been quiet before the attack.

In retaliation, Islamabad has blocked NATO convoys from crossing into Afghanistan, ordered a review of its alliance with the US and is mulling whether to boycott a key conference on Afghanistan next month.

Hundreds of enraged Pakistanis took to the streets Sunday, burning an effigy of President Barack Obama and setting fire to US flags across the country of 167 million where opposition to the government's US alliance is rampant.

Foreign Minister Hina Rabbani Khar telephoned US Secretary of State Hillary Clinton on Sunday to convey a "deep sense of rage" as a joint funeral was held for the dead soldiers, their coffins draped in the national flag.

On the Fox News Sunday talk show, US lawmakers vented frustration over Pakistan, with Republican Senator Jon Kyl demanding Islamabad cooperate with the United States in order to maintain billions of dollars in financial aid.

Senator Dick Durbin, a top Democrat, offered condolences but said US troops were caught in a "diplomatic morass between the incompetence and corruption in Afghanistan, and complicity in parts of Pakistan".

But John Bolton, a former US ambassador to the United Nations, laid bare the dilemma for Washington in handling nuclear-armed Pakistan.

"While it is tempting for many people to say we ought to throw the Pakistanis over the side... as long as that country has nuclear weapons that could fall into the hands of radicals and be a threat worldwide, they have incredible leverage," he said.

The United States in 2009 approved a huge five-year, $7.5 billion civilian assistance package for Pakistan, but some US lawmakers want to cut civilian aid due to concerns over extremism.

burs/jm/mtp