(Adds comment on defending border from military spokesman)

By Sahibzada Bahauddin

KHAR, Pakistan, Sept 16 (Reuters) - Pakistani aircraft bombed militant strongholds in a northwestern region on Tuesday while U.S. drones prowled the sky over another militant outpost on the Afghan border, a military official and residents said.

Pakistani forces launched offensives against al Qaeda and Taliban militants in the northwest in August and the government says hundreds have been killed.

But at the same time, U.S. forces have stepped up strikes on militants on the Pakistani side of the border, angering Islamabad and straining relations between the allies.

Admiral Mike Mullen, chairman of the U.S. Joint Chiefs of staff, arrived in Pakistan on Tuesday night.

Mullen said this month he was not convinced Western forces were winning in Afghanistan and he was "looking at a new, more comprehensive strategy" that would cover both sides of the border, including Pakistan's tribal areas.

Six cross-border U.S. strikes into Pakistan this month, including a bloody ground assault, have infuriated Pakistan and a military spokesman said on Tuesday aggression across the border would be confronted.

Pakistani security officials said on Monday firing by Pakistani troops forced two U.S. military helicopters to turn back to Afghanistan after they crossed into Pakistani territory, although the U.S. and Pakistani militaries denied it.

Pakistani aircraft bombed three areas in the northwestern region of Bajaur on Tuesday as ground troops searched house-to-house for militants, a military spokesman said.

"They have constructed underground bunkers and along with foreign fighters are putting up stiff resistance. Troops are clearing each and every house in these areas," said the spokesman, Major Murad Khan.

Khan had no information about casualties in the Tuesday fighting but the military has said 117 have been killed in Bajaur over the past week. A helicopter gunship killed nine militants in a vehicle, an intelligence official said.

DRONES ON PATROL

An intensifying insurgency in Afghanistan has put pressure on Pakistan to go after militants operating out of sanctuaries in ethnic Pashtun tribal areas on its side of the border.

Worry about Afghanistan's prospects has also led to stepped-up U.S. strikes.

Pakistan's new government has committed itself to the U.S.-led campaign against Islamist militancy even though it is deeply unpopular.

But it objects to cross-border strikes and protested against a helicopter-borne ground assault by U.S. commandos in South Waziristan this month. There have been five U.S. missile strikes this month, killing militants and civilians, officials said.

"If any incursion is made against our soldiers, our checkposts, then we reserve the right to defend them," said military spokesman Major-General Athar Abbas.

Pakistan says U.S. strikes and the civilian casualties they inflict drive people into the arms of the militants.

Prime Minister Yousaf Raza Gilani has said Pakistan aimed to settle the issue with the United States diplomatically.

President Asif Ali Zardari, who is close to the United States, was in Britain on Tuesday where he was due to meet Prime Minister Gordon Brown for talks expected to focus on security.

Zardari was elected president this month to replace staunch U.S. ally Pervez Musharraf, a former army chief who stepped down as president last month under threat of impeachment. (Additional reporting by Haji Mujtaba, Augustine Anthony and Kamran Haider; Writing by Robert Birsel; editing by Robert Hart) (For a blog on Pakistan, see http:/blogs.reuters.com/pakistan/)

AlertNet news is provided by