(Adds reports from local residents, U.S. military comments)

By Hamid Shalizi

KABUL, Aug 16 (Reuters) - U.S. and Afghan air and ground forces pounded al Qaeda militants for a second day on Thursday in the Tora Bora mountains close to the Pakistan border where Osama bin Laden once fled in the wake of the 2001 invasion.

The steep slopes of the mountains are riddled with cave and tunnel complexes built by Afghan and Arab fighters during the 1980s struggle against the Soviet occupation and provide an ideal hideout for guerrilla fighters.

"It is a joint operation conducted by Afghan and U.S. forces, divided by ground and air assets," said Captain Vanessa Bowman, spokeswoman for U.S.-led coalition forces in Afghanistan.

"Afghan and U.S. forces engaged al Qaeda and other violent extremist fighters in the eastern Afghanistan region in Tora Bora," she said, adding that the operation began on Wednesday.

Pakistan has deployed a "limited number" of regular army troops in Kurram tribal region in its side of the Tora Bora range, a security official said.

"It has been done over the past three days and it was done in coordination with allied forces in Afghanistan," he said. "We have made all arrangements to block any infiltration of militants from the other side. So far there has been no attempt of any infiltration."

VILLAGERS FLEE

Afghan media quoted local government officials as saying some 50 militants had been killed in the fighting.

Local residents said dozens of families have fled the area and three villages had been bombed by U.S. and Afghan forces and up to 30 civilians had been killed in the fighting.

The U.S. military said it had no substantiated reports of any civilian casualties.

"We are not targeting any villages and the operation are specifically being conducting away from populated areas," a U.S. spokesman said.

It was not possible to independently verify any casualties.

Aid organisations had suspended projects in the Tora Bora region, said a Western security official in the city of Jalalabad, some 50 km (30 miles) north of the mountains.

"We see a lot of air activity going towards that region, it looks like it's quite intense today," he said.

Three coalition soldiers were killed in a roadside bomb in the Khogiani district near Tora Bora on Sunday and a U.S. base in the area also came under rocket attack in the last few days.

U.S. soldiers and Afghan militia forces launched a major assault on Tora Bora in late 2001 in pursuit of al Qaeda and its leader, Osama bin Laden, who was thought to be hiding in the mountain range after the toppling of the Taliban government.

But U.S. military leaders allowed the Afghan militiamen to spearhead the assault and bin Laden managed to escape.

Al Qaeda forces and their Taliban allies use the rugged and semi-lawless border region between Afghanistan and Pakistan to plan, train and launch attacks in both countries.

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