KABUL (AFP) -US and Afghan troops pressed an air and ground assault against Al-Qaeda militants grouped in the Tora Bora mountains of eastern Afghanistan, the last known hiding place of Osama bin Laden.

Up to 250 families had fled the area near the Pakistan border as the attack pushed into a fourth day, Afghan officials said.

The US military and the Afghan defence ministry would not give details of the operation Thursday as it was still under way.

But US military spokeswoman Captain Vanessa Bowman told AFP it was intended to disrupt Al-Qaeda and other militants who were massing in the region.

Intelligence indicated that the fighters had gathered in dug-in fighting positions, she said.

Bowman said Wednesday, when the US military announced the assault, that the remote and rugged region was an “ideal” area to conceal militant support bases and training sites, as well as plan attacks.

The area, a complex of caves 50 kilometres (30 miles) south of the eastern city of Jalalabad, is known as the last stronghold of Al-Qaeda in Afghanistan.

Bin Laden and fighters from Arab countries built the caves during the 1980s during the resistance to the Soviet occupation of Afghanistan.

Bowman said Wednesday the assault was using precision munitions to avoid civilian casualties.

“The targets were carefully chosen to pinpoint enemy positions and eliminate the likelihood of harming innocent civilians,” she said.

Between 200 to 250 families had fled the Tora Bora valley to other villages, said Mohammad Ali, governor of Nangarhar province’s Pachir Wa Agam district which includes the targeted area.

“We have asked the provincial government for help for the displaced people,” he said.

Afghan media reports said some 50 Taliban had been killed but the governor had no confirmation, saying: “These are only rumours at this stage.”

Al-Qaeda was sheltered by the 1996-2001 Taliban government in Afghanistan and allowed to operate training camps there until the hardline Islamic regime was driven out in a US-led invasion in late 2001.

Bin Laden, the Al-Qaeda chief behind the September 11, 2001 suicide plane attacks in the United States that triggered the invasion, was last seen in the Tora Bora mountains in December that year when US and Afghan forces tried but failed to capture him.

It is widely believed he escaped into the northern tribal areas of Pakistan where intelligence agencies believe he is currently hiding.

The US-led coalition here and Afghan officials have reported the emergence of a new anti-government outfit in the area called the Tora Bora Front.

The shadowy group is believed to be an Al-Qaeda-linked unit set up by the son of Younus Khali, a key commander in the Afghan resistance to the Soviets who later joined the Taliban.

Since its ouster the Taliban has regrouped and is now carrying out daily attacks on allies of the Afghan government, including officials and troops.

In one of the latest incidents, two German police officers and a German foreign ministry employee were killed in Kabul on Wednesday when their convoy was destroyed in a bomb attack claimed by the Taliban.

Meanwhile there were new clashes in the southern province of Helmand where Taliban fighters are said to have hooked up with illegal opium producers who help finance the insurgency.

Four insurgents were killed on Wednesday when coalition war planes bombed compounds and trenches from which insurgents had ambushed troops near the volatile Sangin area, a coalition statement said. No soldiers were hurt.

Nearly 50,000 international troops are working with Afghan security forces to defeat the Taliban’s Al-Qaeda-backed insurgency, with the violence sharply escalating this year.