By Shams Mohmand
PESHAWAR, Pakistan (Reuters) - Pakistan has driven a group of militants from a stronghold in its tribal areas into eastern Afghanistan, the army said on Sunday, in the latest flare-up along the Pakistan-Afghan border.
With each country blaming the other for failing to crack down hard enough on militants, the fighting threatens to raise tensions as the United States prepares to announce a gradual withdrawal of troops from Afghanistan next month.
A Pakistan army statement said it had launched an air and ground assault against a militant stronghold in Mohmand tribal agency and killed 25, while the rest fled across the border.
The area lies opposite the Korengal Valley, from where the United States pulled back its troops in 2010 after deciding to concentrate its forces in population centers in southern Afghanistan, the heartland of the Taliban.
Pakistan complained the withdrawal of U.S. troops from Kunar province opened up safe havens for militants and left it vulnerable to counter-attack after it drove them out of its own tribal areas.
"It was a thickly forested and mountainous area," a paramilitary officer told Reuters. "Whenever we launched operations, militants fled to Afghanistan and then returned and based in the area."
The Pakistani complaints mirror those made by U.S. officials who say fighting in Afghanistan is undercut by militant sanctuaries in Pakistan. It is impossible to obtain independent verification of military activity in the remote border areas.
The latest operation, launched on Saturday, was carried out in the same area where Pakistan on Friday complained that NATO aircraft had attacked one of its military posts after intruding 2.5 km (1.5 miles) inside Pakistani territory.
Pakistan expressed serious concern to the United States over the intrusion. Local officials said a few bombs were dropped but no casualties were reported.
Relations between Pakistan and the United States have been strained since the unilateral raid by U.S. forces who killed Osama bin Laden in the Pakistani town of Abbottabad on May 2.
U.S. forces have also stepped up attacks on militant targets by drone aircraft in the tribal areas and Washington is pushing for the Pakistan army to further expand its operations there.
Pakistani officials say, however, that its military is already overstretched. It also wants a commitment from the United States that its troops will secure its side of the border. Otherwise, it says, its own troops and people become vulnerable to attack by militants based on the Afghan side.
"Almost all of the militant leaders wanted in Pakistan are hiding across the border and they keep sending fighters here to attack our forces and villagers," a senior military official told Reuters.
Last week Pakistani officials said six civilians were killed in a cross-border raid on three villages in Bajaur agency to the north of Mohmand, the second such incursion this month.
On Friday, the foreign ministry summoned the Afghan ambassador to Islamabad and lodged a protest over what it said was a cross-border attack by 100 to 150 militants.
(Additional reporting by Kamran Haider, writing by Myra MacDonald, editing by Angus MacSwan)