Thousands of Pakistani troops, backed by helicopter gunships, have cornered the militants in the mountainous tribal region of South Waziristan. Army spokesman Major-General Shaukat Sultan told reporters the gunmen are well entrenched and putting up fierce resistance. He says efforts are under way to capture as many people alive as possible and prevent them from escaping.

"They would like to break the cordon and break out from the encirclement. That is what they have tried at two or three places. The engagements are mainly with the small arms fire. They [the militants] have also used rockets and they have also used the mortars," he said.

General Sultan says there could be a senior al-Qaida leader among the militants, but he would not confirm reports that the trapped man is Ayman al-Zawahiri, the terror network's number two leader.

"The type of resistance and the type of preparations that we see in the defensive positions, the hardened fortresses that they have made, we can assume that there could be some high-value target there," he said.

The military spokesman says authorities have so far not been able to determine if Mr. Zawahiri is present among the besieged al-Qaida suspects. The Egyptian man is regarded as the brains of al-Qaida.

The Pakistan forces launched the latest anti-terror operation in the tribal region on Tuesday, and since then officials say at least 16 government troops and as many as 32 suspected terrorists, mostly foreigners, have been killed in the fighting. At least 18 suspects, including foreigners, have also been detained.