Pakistan President Gen. Pervez Musharraf said a "high value" target was believed trapped in South Waziristan, a semi-autonomous tribal belt that has resisted outside intervention for centuries.
Hundreds of troops and paramilitary rangers have pounded several fortress-like mud-brick compounds with artillery and fired on them from helicopter gunships, as entrenched suspects fought back hard. An intelligence official said "dozens" were killed Thursday.
CBS News Correspondent Lara Logan reports it's an operation that began several days ago and has met with fierce resistance from tribesmen who're suspected of harboring al Qaeda terrorists.
The intensity of the fighting has led Pakistani officials to believe they may have cornered a top al qaeda leader, Logan reports. The army has surrounded a series of mud-walled forts where the suspects are holding out. Reinforcements are being rushed in and when daylight comes, a new assault is expected, says Logan.
At least 41 people — 15 soldiers and 26 suspected militants — were killed earlier this week in fighting in the area.
Also in Afghanistan Thursday, two American soliders were killed and two more wounded in what U.S. Central Command called anattack in a village near Tarin Kowt. The attack was not believed to be linked to fighting over al-Zawahri.
The region has long been considered the most likely hiding place for the top two al Qaeda leaders — but there was no indication bin Laden was with al-Zawahri. However, the two have traveled together in the past, and bin Laden and al-Zawahri appeared jointly in video tapes released shortly after the Sept. 11, 2001, attacks on the United States.
The United States has offered a U.S. $25 million reward for information leading to al-Zawahri's capture. On Thursday, the U.S. House of Representatives doubled the reward for bin Laden's capture to US$50 million.
"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," a Pakistani military official said. "All of our efforts are to capture him."
An intelligence official and a senior politician in Musharraf's government both confirmed the account. All spoke on condition of anonymity.
The intelligence official said information was also coming from some of the 18 suspects captured during Thursday's operation. Some said during interrogation that al-Zawahri was wounded in the raid, the official said. Officials said helicopter gunships and artillery would continue attacking at dawn Friday.
Musharraf told CNN that he'd spoken with the commander of Pakistani troops in the region. He said the commander reported "fierce resistance" from a group of fighters entrenched in fortress-like buildings, and that there were indications a senior figure was surrounded.
"He's reasonably sure there's a high-value target there," Musharraf said. "They are not coming out in spite of the fact that we pounded them with artillery."
The news came the same day as U.S. Secretary of State Colin Powell announced in the capital, Islamabad, that Washington was bestowing the status of "major non-NATO ally" on Pakistan, and praised the country for its help in the war on terror.
U.S. National security adviser Condoleezza Rice told CNN she could not confirm the reports.
But, she said, if al-Zawahri were captured, "it would be of course a major step forward in the war on terrorism, because he's obviously an extremely important figure. But I think we have to be careful not to assume that getting one al Qaeda leader is going to break up the organization."
The 52-year-old former Egyptian surgeon is believed to be the brains behind the terror network, with bin Laden serving more as spiritual leader and financial backer.
Often seen by bin Laden's side in videos released to Arab television networks, the doctor was also thought to serve as al Qaeda leader's personal physician.
Al-Zawahri's Egyptian Islamic Jihad was believed behind the assassination of President Anwar Sadat during a Cairo military parade in 1981. He merged the organization with al Qaeda in 1998.
CBS National Security Correspondent David Martin reports experts who have researched Zawahri's life say he was a
In 1998, when bin Laden issued his now infamous order that it was the duty of every Muslim to kill Americans, both military and civilian, Zawahri was one of the signers. That same year, reports Martin, al Qaeda suicide bombers blew up two American embassies in Africa.
Al-Zawahri has continued to spread his message since the Sept. 11 attacks in audiotapes, the latest broadcast on Feb. 24, in which he taunted U.S. President George W. Bush and threatened more attacks on the United States. Another tape criticized France's decision to ban Islamic headscarves in schools.
Under pressure from Washington, Pakistan has arrested more than 500 al Qaeda suspects and has turned most over to the United States. The last major capture was that of Khalid Shaikh Mohammed, the former al Qaeda No. 3, who was nabbed on March 1, 2003, in a house near the capital and quickly delivered to U.S. custody. He is being held at an undisclosed location.
The Pakistani military has been pursuing 100 tribal leaders whom authorities want to roll into their efforts to hunt al Qaeda in the Waziristan frontier. So far, about two-thirds have said they would provide information and turn over any Islamic militants in their territories, American defense officials said.
The others face destruction of their homes by the Pakistani military, officials said.
U.S. officials say they are watching to see if the Pakistani actions send militants back into Afghanistan, where U.S. troops operate freely. The U.S. military on Sunday announced the start of a new operation to track down senior al Qaeda and Taliban fugitives.
Two American soldiers were killed and two others were wounded in fighting Thursday in central Afghanistan, the U.S. military said. At least five attackers were killed in the battle. The military said that the fighting did not appear to be directly related to the siege against al-Zawahri.
Powell, who left the country hours before the news broke, also said he believed there was evidence that bin Laden is hiding in the rugged border area.
"No one has seen him, so how can one be sure?" Powell told Geo TV. "If he is alive and active, and the evidence suggests that he is, and if he is in the area of the Pakistan-Afghan border, that's a very difficult area to find someone who doesn't want to be found."
— Associated Press reporters Katherine Pfleger Shrader in Washington, Munir Ahmad in Islamabad and Riaz Khan in Peshawar, Pakistan, contributed to this report.