Pakistan's military forces have continued to pound suspected hideouts of the al-Qaida terror network near the Afghan border. The militants are said to be putting up fierce resistance, leading to suggestions they may be trying to protect a senior al-Qaida leader.

The clashes erupted Tuesday when Pakistani forces surrounded a cluster of villages in the country's mountainous tribal region known as South Waziristan. Dozens of suspected al-Qaida militants - mostly foreigners - are believed hiding in the area, a safe haven for fugitives.

Authorities and witnesses say Pakistani troops have used heavy artillery and helicopter gunships to attack the militants, who are reportedly well entrenched and putting up stiff resistance.

Military spokesman Major-General Shaukat Sultan says the troops are determined to flush out terrorists.

"The firefight is still going on," he said. "It is on. The operation is on."

Senior government officials believe the gunmen, mostly Chechens and of Central Asian origin, may be defending what they call a high-value target. Observers say the suspected militants could be fighting a do-or-die battle because Pakistani forces are surrounding them.

"It is a question of their life and death struggle combined with also the protection of certain high-value targets," said Talat Masood, a former Pakistan general. "That is the reason they are putting up such a stiff resistance."

Pakistani and U.S. counter-terrorism officials have refused to confirm media reports that al-Qaida's number two leader, Ayman al-Zawahiri is among the gunmen fighting government forces.

As many as 45 people, including at least 15 army and paramilitary troops, have been killed in the fighting. A significant number of Pakistani security personnel are reported missing and there are fears they may have been taken hostage. Authorities have detained at least 18 suspects.

U.S. and Afghan forces have reportedly strengthened their positions on the Afghan side of the border to prevent al-Qaida militants from escaping the Pakistani manhunt.