By Robert Birsel

ISLAMABAD, Jan 17 (Reuters) - Pakistani security forces were searching on Thursday for 15 paramilitary soldiers who went missing when hundreds of militants stormed a fort in the northwest of the country, a military spokesman said.

Militants have been blamed for a string of attacks on security forces in recent months, compounding a sense of crisis in the nuclear-armed country as President Pervez Musharraf has struggled to hold power in the face of protests from opponents.

In the latest incident, unidentified attackers fired three rockets near a Pakistani air force engineering base 75 km (45 miles) northwest of the capital, Islamabad, on Thursday but no one was hurt, military officials said.

Early on Wednesday, about 200 militants attacked the Sara Rogha fort in South Waziristan on the Afghan border. They captured it after blowing up one of its walls, the military said.

The military initially said 40 militants and seven soldiers were killed and 20 soldiers were missing, but five of the missing men were later reported to have reached villages in the area, said military spokesman Major-General Athar Abbas.

"Fifteen are still missing," Abbas said. "They are looking for the men, a search operation is on."

A spokesman for the militants said on Wednesday they had killed 16 troops and captured 12 in the attack. He said only two of his men were killed.

Abbas said there was no confirmation of the militants' claim that they had captured the 12.

In a major embarrassment for the military, militants captured about 250 soldiers in the same region in August and held most for more than two months before releasing them. The militants executed several of the men.

MILITARY PREPARING OPERATION

Abbas said the militants had abandoned the fort and security forces were preparing an operation to take control of it again.

Security forces have been battling al Qaeda-linked militants in South Waziristan for several years.

The area where the fort is located is a stronghold of al Qaeda-linked militant leader Baitullah Mehsud, who the government said was behind the assassination of opposition leader Benazir Bhutto in Rawalpindi on Dec. 27.

The government says the militants are intent on destabilising the country in the run-up to a Feb. 18 general election that is meant to complete a transition to civilian rule.

The parliamentary elections were due to be held on Jan. 8 but were postponed after Bhutto was killed in a gun and bomb attack as she left an election rally.

Militants flocked to Waziristan and other parts of the Afghan-Pakistani border in the 1980s to support Afghan guerrillas fighting Soviet troops in Afghanistan.

Many al Qaeda and Taliban members took refuge on the Pakistani side of the border after U.S.-led troops ousted the Taliban government in Afghanistan weeks after the Sept. 11, 2001, attacks on the United States.

They also launch raids into Afghanistan from border sanctuaries.

As part of its efforts in the U.S.-led campaign against terrorism, Pakistan has been trying to expel foreign militants and subdue their Pakistani allies on the lawless border, where Osama bin Laden is believed to be hiding.

All three rockets fired towards the air force base at Kamra landed on open ground and caused no damage or injuries, an air force spokesman said.

Last month, a suicide bomber rammed a car packed with explosives into a school bus at the same base, killing himself and wounding nine people, including several children. (Additional reporting by Kamran Haider; Editing by Alex Richardson)

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