NEW DELHI, India ? Pakistani officials said Thursday they had cornered Osama bin Laden’s top lieutenant, Ayman al-Zawahri, in a remote location near the Afghan border.

After two days of intense fighting, the officials said in Islamabad that the man regarded as the “brains” behind the al-Qaida network had been surrounded in the tribal area of South Waziristan bordering Afghanistan.

The reports were highly speculative and could not be independently confirmed. But for the past two days, Pakistani troops have been locked in combat with a group of tribesmen and suspected foreign fighters in a remote region of the tribal frontier that has long been regarded as a sanctuary for fugitive al-Qaida fighters.

At least 41 people, including 15 Pakistani soldiers and 26 militants, have been killed since the large-scale military operation was launched Tuesday against local tribesmen suspected of harboring al-Qaida fugitives, according to Pakistani military officials.

In an interview with CNN, President Pervez Musharraf said the ferocity of resistance encountered by the Pakistani soldiers had led military officials to believe that the fighters must be shielding a “high-value target.”

Intelligence officials quoted by The Associated Press said the target was al-Zawahri, a former Egyptian doctor who has spent most of the past two decades at bin Laden’s side.

“We have been receiving intelligence and information from our agents who are working in the tribal areas that al-Zawahri could be among the people hiding there,” one military official said.

If al-Zawahri were caught, it would mark a milestone in the terror war. Although bin Laden remains the undisputed leader of the increasingly fragmented al-Qaida network, his role is seen as largely symbolic and inspirational. U.S. officials say they aren’t even sure he is alive.

Al-Zawahri, who is regarded as the network’s chief operator, is the man who supervised the plotting of the Sept. 11, 2001, attacks and who has continued to direct al-Qaida’s activities since the collapse of the Taliban deprived al-Qaida of its operational base in Afghanistan.

U.S. officials say they are certain al-Zawahri is alive and hiding out along the Afghanistan-Pakistan border, where he has been organizing continued resistance against U.S. forces in Afghanistan and probably also globally.

Although it is thought unlikely that any of the al-Qaida leaders still hiding in the remote wilderness of the border region are able to exert any direct day-to-day control over terrorist operations elsewhere, al-Zawahri has issued numerous tape-recorded messages urging al-Qaida followers to continue to attack Americans around the world. In one taped message in September, he called on Pakistanis to kill Musharraf, who narrowly escaped two assassination attempts in December.

Musharraf blamed al-Qaida for the attempts on his life and has since shown a renewed interest in cracking down on the network. Starting in January, Pakistani troops have moved in force into the tribal areas, where they have been attempting to flush out the al-Qaida fighters hiding there by putting pressure on the local tribesmen suspected of sheltering them.

The latest operation coincided with a visit to Islamabad by Secretary of State Colin Powell, who promised to upgrade America’s military relationship with Pakistan.

U.S. forces based on the Afghan side of the border also have been stepping up their operations in what the commander of U.S. forces in Afghanistan, Lt. Gen. David Barno, describes as a “hammer and anvil” tactic designed to squeeze al-Qaida and Taliban fighters between the armies of the United States and Pakistan deployed on either side of the border.

The United States is offering a $25 million reward for al-Zawahri’s capture, and the House voted Thursday to double the reward for bin Laden to $50 million. That bill, which passed 414-0, now goes to the Senate.