Pakistan is redeploying some of its border forces to an area opposite Afghanistan's Helmand province to block any Taliban trying to flee from a U.S. offensive launched there on Thursday, a military spokesman said.

Thousands of U.S. Marines stormed into a river valley in the southern Afghan province of Helmand province in an operation seeking to break the Taliban's hold on the opium-growing region and turn the tide of the war in Afghanistan.

Southern Helmand shares a 200-km (130-mile) desert border with the southwestern Pakistani province of Baluchistan and troops were being moved there to "challenge any crossing", said a Pakistani military spokesman.

"It's a reorganisation of the deployed troops on the border," said the spokesman, Major-General Athar Abbas.

"The area which is not under stress at the moment, we can pull out troops from those areas and beef-up the area which is coming under effect," he said.

He did not say how many soldiers were involved.

The U.S. offensive in Afghanistan comes as Pakistani soldiers are battling Pakistani Taliban in the Swat valley, northwest of Islamabad, and in South Waziristan, a militant enclave opposite eastern Afghanistan.

Abbas said there was no question of Pakistani soldiers crossing the border to help U.S. forces.

"We're only strengthening our border posts, we're not moving into Afghanistan," he said.

For now, the U.S. offensive is centred on the Helmand river valley, in an area a long way from the Pakistani border and there have been no reports of fighting near the border.