WANA, Pakistan - Thousands of Pakistani army reinforcements joined a major offensive today in tribal border villages where al-Qaida's No. 2 leader, Ayman al-Zawahri and hundreds of other militants are believed surrounded, while Afghan authorities reported the arrests of midlevel terrorist leaders on their side of the border.

Army spokesman Gen. Shaukat Sultan said the army believes some 300-400 militants - a mix of foreigners and local Pakistani tribesmen - are holed up in fortresses in several villages in lawless South Waziristan, where Pakistani paramilitary forces began an operation against al-Qaida and Taliban fugitives four days ago.

"From the type of resistance we are getting ... the militants could be anything from 300 to 400," he told a news conference.

Sultan said authorities were convinced a high-level fugitive was among the fighters, though he said it was unclear whether the man was al-Zawahri.

"The type of resistance, the type of preparation of their defensive positions, the hardened fortresses they have made means we can assume that there could probably be some high-value target there," Sultan said from the army press office in Rawalpindi, a city near the capital, Islamabad.

But he disputed claims by four senior Pakistani officials that captured militants had revealed that al-Zawahri was among them, and possibly injured.

"So far, whatever people we have apprehended, we have not got confirmation from them," he said, but added: "Even if we knew more, we couldn't tell you."

Villagers in Wana, the main town in South Waziristan, said heavy guns fired through the night and jet fighters were visible in the area as fighting spread today to two more tribal villages.

Helicopter gunships fired rockets at houses in Shin Warsak, five miles southwest of Wana, said the villagers, who were streaming out of the besieged region in pickup trucks loaded with families and possessions.

Residents reported seeing scores of army trucks carrying troops and weapons moving from Wana to the target areas.

Townspeople said heavy guns fired through the night and jet fighters were visible in the area, as fighting spread today to two more villages in South Waziristan, where Pakistani paramilitary forces began an operation against al-Qaida and Taliban fugitives four days ago.

Helicopter gunships fired rockets at houses in Shin Warsak, five miles southwest of the main town of Wana, said the villagers, who were streaming out of the besieged region in pickup trucks loaded with families and possessions.

Residents reported seeing scores of army trucks, carrying troops and weapons, including light artillery and heavy machine-guns, and some armored vehicles. Army troops took up positions on rooftops to provide security for the convoy of troops moving from Wana to the target areas near the Afghan border.

U.S. national security adviser Condoleezza Rice said a "fierce battle was raging" but the United States did not have any independent confirmation that al-Zawahiri is surrounded.

Asked if the United States was dismissing the Pakistani claim, she said, "no, not at all. I think the Pakistanis know their business."

Gen. Richard Myers, chairman of the joint chiefs of staff, also responded cautiously, but added al-Zawahri's capture would be a major victory.

"By itself it's not going to stop plots that are already under way," Myers told CNN. "But when you take the head off an organization it's obviously going to have an impact."

Meanwhile, a spokesman for Afghan President Hamid Karzai said U.S. and Afghan troops have captured "semi-senior" terrorist leaders along the border with Pakistan, as they tightened security along the rugged frontier.

Presidential spokesman Jawed Ludin said it was unclear if those detained had fled the battle in Pakistan, and declined to give any details of who might be in custody.

"Some of the arrests have included semi-senior leadership within the terrorist elements on the Afghan side, possibly with strong links to al-Qaida," he told AP in Kabul.

Villagers around Wana reported a lull in the fighting today amid reports that tribal elders were trying to mediate with local authorities and establish a cease-fire.

Pakistani forces have battled with hundreds of militants in at least five villages near Wana, pounding fortress-like mud-brick compounds. An intelligence official said "dozens" were killed Thursday.

At least 41 people - 15 soldiers and 26 suspected militants - were killed earlier this week in fighting in the area.

This semiautonomous tribal region, which has resisted outside control for centuries, has long been considered a likely hiding place for the top two al-Qaida leaders - but there was no indication bin Laden was with the Egyptian-born 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.

The United States has offered a $25 million reward for information leading to al-Zawahri's capture. On Thursday, the House of Representatives doubled the reward for bin Laden's capture to $50 million.

Pakistani President Gen. Pervez Musharraf

Musharraf told CNN that he had 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."

Al-Zawahri, a 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. He is also thought to have served as the al-Qaida leader's personal physician.

He has often been seen at bin Laden's side in videos released to Arab television networks. In a broadcast on Feb. 24, al-Zawahri taunted President Bush and threatened more attacks on the United States.

Under pressure from Washington, Pakistan has arrested more than 500 al-Qaida suspects and has turned most over to the United States. The last major capture was that of Khalid Shaikh Mohammed, the suspected former al-Qaida No. 3, who was nabbed on March 1, 2003, in a house near the capital and quickly delivered to U.S. custody.

Brig. Mahmood Shah, the chief of security in Pakistan's tribal areas, estimated that between 300 and 500 al-Qaida fugitives and supporters were in the area. He said the suspects were using light and heavy weapons, and could not forecast how long the operation would last.

"Our effort is to take this operation to its logical end," he said. "It may take some time."

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-Qaida and Taliban fugitives.