(CBS/AP) A suicide bomber blew himself up at an army mess hall in northwestern Pakistan on Thursday, killing at least 15 soldiers, officials said.

Militants, meanwhile, attacked a base and triggered a battle that left about 50 insurgents and two soldiers dead in the South Waziristan region near the Afghan border where pro-Taliban fighters hold sway, the army said.

The violence came as visiting U.S. Deputy Secretary of State John D. Negroponte met with President Gen. Pervez Musharraf in Islamabad.

The attack at the mess hall in Ghazi Tarbela, an army facility about 60 miles south of the capital of Islamabad, was the work of a suicide bomber, according to two security officials, who spoke on condition of anonymity because of the sensitive nature of their job.

Maj. Gen. Waheed Arshad, the spokesman for Pakistan's army, confirmed to Pakistan's private Geo news channels that 15 soldiers were killed and 11 wounded, some seriously, but he would not say what caused the blast.

"We are trying to establish what may have caused this incident," Arshad told CBS News' Fahan Bokhari.

The two intelligence officials also said the victims belonged to the army's Karar commando group, which has participated in operations against militants.

Arshad said the army forces in South Waziristan repelled repeated militant attacks in Razmak. Army helicopters and ground fire destroyed four rebel positions, he said.

Arshad said their initial estimate was that at least 30 militants were killed, but added later that tribesmen informed officials that as many as 50 insurgents had died in the military attack. He said two soldiers had died and eight were wounded.

A militant rocket hit a transformer and power line, cutting electricity in Razmak, he said.

The blast marked the second time this month that the army has been targeted. On Sept. 4, two suicide bombers struck targets in Rawalpindi, the military's main garrison town outside Islamabad, the Pakistani capital, leaving at least 29 people dead.

One of those targets was a bus belonging to the InterServices Intelligence, which was carrying the agency's junior employees. No one immediately claimed responsibility for Sunday's blast.

But a senior western diplomat who spoke to CBS News on the condition of anonymity said the attacks could be in retaliation for Pakistan's ongoing military operations against al Qaeda and Taliban in the country's border regions along the Afghan border.

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