U.S. forces say they have detected the voice of Usama bin Laden over short-range radio.

A U.S. official, speaking on condition of anonymity, revealed Saturday that radio messages issued by the suspected terrorist leader were intercepted in the Tora Bora region of eastern Afghanistan. The discovery has convinced U.S. military officials that bin Laden is probably in the area, where Al Qaeda forces are attempting to fend off Afghan fighters aided by American and British special forces.

Leaders of the Afghan fighters believe bin Laden is trapped in a cave in the snowy, rugged mountains.

Authorities compared the voice on the radio to videotape recordings of bin Laden’s voice in order to determine that it was his, according to the U.S. official. Americans have many ways to eavesdrop on such radio messages, including equipment carried by special operations forces and instruments on aircraft and satellites.

Afghan allies of the United States have also captured about 50 Al Qaeda fighters after a bout of heavy combat in the area, Defense Secretary Donald H. Rumsfeld said.

U.S. troops, along with Afghan fighters, have searched caves captured from Al Qaeda for documents and other evidence to track down other members of the group and link them to acts of terrorism. Americans have done the same at Al Qaeda sites captured earlier in the Afghanistan campaign.

Such recovered material, coupled with information gathered elsewhere in the terrorism investigation, has helped facilitate the arrest of Al Qaeda operatives in foreign countries, said the other U.S. official, who also spoke on condition of anonymity.

The Defense Secretary, while visiting Azerbaijan to discuss military ties, said Saturday it was too early to determine whether operations around Tora Bora area would end soon. He also said information gathered from the interrogation of prisoners is "accumulated and correlated, judgments are be made as to its accuracy or a lack of accuracy, and then we shall proceed to next steps."

While Afghan leaders said they believe top suspect Usama bin Laden is trapped in the area, but Army Gen. Tommy Franks said the U.S. military is not sure where he is.

The military guesses that between 300 and 1,000 Al Qaeda fighters are trapped in the two valleys, Franks told reporters near Central Command headquarters in Tampa, Fla., and at the Pentagon via two-way video. The Al Qaeda members are dispersed on valley floors, in fortified mountainside caves and along high ridges, he said.

Rumsfeld said on his trip to Central Asia that "we think there are not a lot of ways out" for Al Qaeda fighters.

Franks said U.S. soldiers have fought on the front lines in the rugged mountain region. He said he did not know of any U.S. soldiers wounded in the battles, although some members of the international anti-terrorism coalition have sustained injuries.

Opposition fighters told The Associated Press on Friday that two Americans had been grazed by machine-gun bullets in the Tora Bora area. British special forces troops also are in the area.

Franks said John Walker, the American Taliban fighter who surrendered to American forces, has been transported to the USS Peleliu in the Arabian Sea. Walker had been detained at Camp Rhino, the remote U.S. Marine desert base 70 miles from the southern Afghan city of Kandahar. U.S. officials have not decided whether to keep Walker in military custody or turn him over to the civilian justice system, Franks said.

Along with bin Laden, the U.S. military does not know the location of the Taliban's leader, Mullah Mohammed Omar, Franks said. Omar has disappeared after being in Kandahar when the city fell last week.

Many Al Qaeda members have been killed in this week's fighting at Tora Bora, but the United States has not determined if any of them are senior leaders, Franks said.

Three high-ranking Al Qaeda leaders are known to be dead, all killed by U.S. airstrikes last month: Egyptians Mohammed Atef, Tariq Anwar al-Sayyid Ahmad and Muhammad Salah.

"We probably have killed some people on this (most wanted) list and don't know it yet," Franks said.

Captured Al Qaeda and Taliban leaders could be held by U.S. forces at Camp Rhino like Walker was, at the new Marine outpost at Kandahar airport, or possibly other places in Afghanistan, Franks said. He said prisoners also could be held, like Walker, on U.S. Navy ships.

The Associated Press contributed to this report.