Authorities in Pakistan say some suspected members of the al-Qaida terror network may have escaped a weeklong siege near the Afghan border through a series of secret tunnels. Thousands of Pakistani troops have surrounded the heavily armed militants, many of the foreigners, in a remote mountainous tribal region known as South Waziristan.

The chief of security for Pakistan's semiautonomous tribal regions, Brigadier Mehmood Shah, says troops have discovered a two-kilometer tunnel through which top al-Qaida suspects may have escaped. He told reporters Monday the tunnel connects mud homes of two wanted local tribesmen before ending in a dried-up stream near the Afghan border.

Hundreds of foreign militants and their local supporters are fighting the government forces with rockets, machine guns and grenades. Some 5,000 Pakistani troops backed by helicopter gun ships are involved in the operation that has left dozens of people dead, including a significant number of Pakistani troops.

Military officials say the strong resistance put by the militants indicates they are protecting some senior al-Qaida suspects, possibly the terror network's number two leader, Ayman al-Zawahiri. Pakistani experts are conducting DNA tests to try to identify those killed or still detained in the operation. Pakistani military spokesman Major General Shaukat Sultan declined to elaborate on whether they include any important terror figure such as the Egyptian-born al-Zawahiri.

"The tests would be matched with all the available information, either it is already available or it would be collected," said General Sultan. "At this moment whatever information we have about the dead or the alive, we would not like to give out that information till the time we are 200-percent sure who they are."

Pakistani officials say the prisoners include Pakistanis, Arabs, Chechens and Islamic militants from central Asian republic of Uzbekistan. In charge of the military operation, Lt. General Safdar Hussain says the suspected terrorists are well entrenched and have shown no signs of giving up.

"These people are highly determined," stressed General Hussain. "The way these people are fighting, I think its going to be a very long-drawn battle, and I believe that they are determined to fight last man, last round. And we are determined to make sure that either they are killed or apprehended."

The military campaign against suspected al-Qaida militants is the biggest Pakistan has waged since joining the U.S.-led war on terrorism.