TORA BORA, Afghanistan — Afghan tribal commanders set a new deadline Wednesday for the surrender of a group of al-Qaida fighters cornered in a mountain canyon under heavy U.S. bombardment, demanding that top terrorist suspects, possibly including Osama bin Laden, also turn themselves in.

U.S. airstrikes sent smoke and debris billowing into the air as they pounded the desolate canyon where the al-Qaida fighters were boxed in after being forced out of their caves in the Tora Bora region of eastern Afghanistan. An earlier deadline of Wednesday morning passed without the men's surrender.

Saying they wanted to end the carnage, tribal eastern alliance leaders gave a new ultimatum, giving the men from bin Laden's terror network until noon Thursday (12:30 a.m MST Thursday) to surrender. But the alliance said it would not accept their surrender unless any top leaders with them also turned themselves in.

This would include bin Laden and others on Washington's list of most wanted terrorist suspects, if they were at Tora Bora, said Ghafar, the mayor of the nearby city of Jalalabad.

"But we don't know where Osama is," said Ghafar, who like some Afghans goes by one name only. He said the alleged terrorist chief might be hiding in thick alpine forest along the nearby border with Pakistan. Other leaders might have escaped during a failed cease-fire overnight, he said.

The Pentagon underlined that it doesn't want to see a deal that lets al-Qaida leaders go free. "We have made it very clear what our intents are," Pentagon spokeswoman Victoria Clarke said Wednesday. Some U.S. officials and Afghan tribal leaders suspect bin Laden is in Tora Bora, though others believe he is hiding in southern Afghanistan.

Tora Bora — an extensive network of caves and tunnels in the White Mountains near the Pakistani border — is the last "effective" stronghold of al-Qaida, according to Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff.

Tribal alliance fighters overran some of the thousands of caves in the area on Tuesday, driving out an undetermined number of al-Qaida fighters — mostly Arabs and other non-Afghans — who became trapped in the canyon and asked to be allowed to surrender.

The eastern alliance agreed to pause fighting until Wednesday morning to surrender unconditionally. The cease-fire fell apart, apparently after the trapped fighters set new surrender terms. U.S. warplanes kept up attacks even during the Afghans' pause, with at least one American AC-130 gunship heard hitting the area overnight.

After the Wednesday morning deadline passed, circling B-52s and other U.S. warplanes appeared from a distance to directly hit the canyon. Others appeared to fall on al-Qaida positions nearby in the Tora Bora and Milawa valleys, which are riddled with hundreds of caves and tunnels. There were no immediate reports of casualties. Ghafar said the battle Wednesday night was focused in an area of about 8 to 12 square miles

Witnesses said 60 men, who appeared to be American personnel, were seen near the front line Wednesday. They were wearing Afghan shawls and floppy pacole caps but were carrying what the witnesses said were U.S.-made weapons and backpacks. An alliance subcommander said 40 British special operations troops also were fighting in the valleys.

Ghafar confirmed the presence of Western troops but said they were acting as observers or guides for U.S. warplanes.

Meanwhile, more U.S. agencies have arrived at a Marine camp in southern Afghanistan to question its only prisoner — an American who joined the Taliban, officials said Wednesday.

John Walker, 20, of Fairfax, Calif., was found among Taliban fighters held at a fortress in northern Afghanistan after an uprising by the prisoners was put down in late November. U.S. officials have not decided how his case will be handled.

He already has been questioned by the CIA. Marine Capt. Dave Romley confirmed that other, unspecified agencies have come to see Walker, who is being held in a heavily guarded green metal shipping container. The Marines are feeding and providing security for Walker and treating a gunshot wound he suffered, Romley said.

Walker has sent a letter to his parents saying he is safe and regrets not contacting them sooner.

Walker apparently dictated the letter to an International Red Cross volunteer.

The lawyer for Walker's parents e-mailed a copy of the letter, dated Dec. 3, to The Associated Press on Tuesday.

"I apologize for not contacting you in such a long time. I realize this must have caused you a lot of g(rief)," reads the letter, addressed to "Dear Papa and Mama." The law firm said it inserted the words in parentheses because they were hard to read on the fax.

"I am currently alive and well in Afghanistan and I am in safe hands," Walker continued. "I cannot give you many details a(bout) my situation but it would be g(ood) to hear from you all."

Neither of the parents, Frank Lindh and Marilyn Walker, nor their lawyer were available for comment Tuesday.

The Washington Times, citing U.S. intelligence officials, reported Wednesday that Walker has told U.S. intelligence officials al-Qaida plans to launch a biological attack against the United States within days.

Walker told intelligence debriefers that "Phase II" of al Qaida's war on the United States would occur at the end of the Muslim holy month of Ramadan, the report said.

Ramadan ends Sunday.

U.S. officials say they have little reason to believe he would have specific information about impending terrorism.

The overnight pause in fighting fell apart after the Pakistan-based Afghan Islamic Press news agency reported that the trapped foreigners demanded that diplomats from their home nations be present for their surrender.

They also wanted to be handed over to the United Nations, something the alliance had reportedly promised. The agency estimated the size of the force at 1,000 men from Sudan, Saudi Arabia, Libya, Egypt, the United Arab Emirates, Kuwait, Yemen, Iraq and Chechnya. The report could not be independently verified.

Pentagon officials warned that some al-Qaida members might still be hiding in the underground Tora Bora network. Defense Secretary Donald H. Rumsfeld said holdouts might continue to fight and that "a wounded animal can be dangerous."

For days, U.S. planes have bombed relentlessly, including 15,000-pound "daisy cutters" that flattened hillsides and left massive craters. Myers said American troops entered some caves, but he did not know if they had been used by al-Qaida.

Gen. Tommy Franks, the U.S. commander of the war, sent at least several Air Force AC-130 gunships to the Tora Bora area to team with Predator unmanned surveillance planes. Predators can feed real-time video of ground activity to the gunships, which are capable of unleashed devastating attacks on ground targets with their 40mm cannon, 105mm howitzer, and 25mm Gatling gun.

The 1,300 Marines on the ground in Afghanistan for the last 18 days have increased their patrols around the southern city of Kandahar, the Taliban's former spiritual capital.

In Stockholm, U.N. Secretary-General Kofi Annan called for the quick deployment of a multinational peacekeeping force in Afghanistan and said he hoped Britain would agree to lead it. However, he was not certain that the force could be in place by Dec. 22 when an interim administration takes office in the capital, Kabul.