Pakistani President Pervez Musharraf told CNN the fierce resistance his troops were encountering led commanders in the field to believe al-Qaida fighters were shielding a "high-value target."
"We have been receiving intelligence and information from our agents who are working in the tribal areas that al-Zawahiri could be among the people hiding there," a military official said, according to the Associated Press. "All of our efforts are to capture him."
The operation, which began Tuesday in South Waziristan with hundreds of troops and paramilitary rangers firing artillery and using helicopter gunships, resulted in the death of dozens of al-Qaida fighters and the capture of 18, some of whom have said al-Zawahiri has been wounded, the AP reported.
Al-Zawahiri, an Egyptian doctor, is considered the "brains" of al-Qaida. The United States has offered a $25 million reward for information leading to his capture.
The Pakistani military has been pursuing 100 tribal leaders, Arabs and other foreigners who settled in the tribal regions after the Afghan-Soviet war in the 1980s and may be providing a support network for bin Laden and his followers.
If the resistance at the border continues, the Pakistani government reportedly may respond with an airstrike Friday morning.
"The militants appear to be well dug in, they're very well prepared and they're determined to fight till the last," said Pakistani military spokesman Maj. Gen. Shaukat Sultan, according to Reuters.
U.S. troops have been battling al-Qaida fighters and their allies on the Afghan side of the border in what the Pentagon is calling a "hammer and anvil" operation.
At least 41 people, including 15 soldiers and 26 suspected militants, were killed Tuesday during fighting in the area, according to the AP.
Secretary of State Colin Powell, who was in Pakistan Thursday to assure the country of U.S. military support, said Washington was giving Pakistan the status of "major non-NATO ally" for its help in the war on terrorism.