The Taliban confirmed on Saturday that U.S.-led airstrikes in Afghanistan scored their biggest success to date, killing one of the top men in Usama bin Laden's Al Qaeda network.

Mohammed Atef was Al Qaeda's military chief, and his death along with that of seven colleagues three days ago is a potentially crippling wound to the terrorist organization the U.S. believes to have been behind the Sept. 11 attacks on America.

A 57-year-old Egyptian, Atef wrote Al Qaeda's terrorism manual, ran the organization's training camps and was suspected of helping plan the attacks.

Meanwhile, Taliban supreme leader Mullah Mohammed Omar was reportedly prepared to flee his southern stronghold of Kandahar in what would be a major blow for the Islamic militia that ruled the Central Asian country.

The rapidly changing situation in Afghanistan was also highlighted by the arrival in Kabul Saturday of Omar's counterpart in the Northern Alliance.

Former Afghan president Burhanuddin Rabbani had not set foot in the Afghan capital for the first time since he was driven from power by the Taliban in 1996.

President George W. Bush decided to launch airstrikes on Afghanistan after the Taliban refused to hand over bin Laden and the Al Qaeda militants.

Mullah Najibullah, a Taliban official in the southeast Afghan border town of Spinboldak, confirmed Atef's death but would not identify the location of the airstrike or the other Al Qaeda members who died with him.

It was the first time a senior Taliban official has confirmed Friday's claim by U.S. officials that Atef was killed. Those officials said Atef was struck in a bombing raid outside Kabul.

An Expert in Terrorism

Atef was a seasoned hand at terrorism whose resume reads like that of a CEO of destruction, and who is suspected of helping coordinate the attacks on the World Trade Center and Pentagon. He directly planned the embassy bombings in Africa in 1998 that killed 224 people, according to a U.S. indictment that charged him with murder, and also was accused of helping plan a 1993 helicopter shootdown in Somalia that killed 18 U.S. soldiers.

A close confidant of bin Laden, whose daughter was married to bin Laden's son, he was often seen at bin Laden's side in photographs and videotapes taken in Afghanistan in recent years.

He came to prominence after he moved from Egypt to Afghanistan in the mid-1980s to join Afghan guerrillas fighting Soviet forces. After the Soviets pulled out, he joined Al Qaeda.

Days after the U.S began its airstrikes on Afghanistan, Atef warned that U.S. troops would suffer the same fate in Afghanistan as they did in Somalia.

"America will not realize its miscalculations until its soldiers are dragged in Afghanistan like they were in Somalia," Atef was quoted as saying by the London-based Islamic Observation Center.

Will Omar Stay or Will He Go?

The U.S. and its allies could chalk up another success if Omar really is willing to leave his headquarters in the southern city of Kandahar in the face of withering U.S. bombing and opposition Northern Alliance gains.

The Afghan Islamic Press reported Friday that Omar had agreed to leave the city within 24 hours and would head for the mountains, turning Kandahar over to local leaders from Pashtun tribes – ethnic kin to the Taliban leaders

But American officials were skeptical of the report.

"I don't put much stock in at this point. I don't believe it," Pentagon spokesman Rear Adm. John Stufflebeem, said.

A Taliban diplomat denied the report by the AIP, a news agency that has good contacts with the Taliban.

"It's completely wrong, it's a baseless report," said Abdullah Hamad, the consul general in Quetta, about 120 miles southeast of Kandahar.

If Omar and the Taliban were to leave Kandahar, it would allow the militia to leave the city to Taliban-friendly Pashtuns.

But on the other hand, it would leave the Taliban without any major cities under their control, making them essentially a guerrilla force in the hills of southern Afghanistan.

Preparing for a New Afghanistan

On the political front, Rabbani's arrival in Kabul marked the beginning in earnest of the planning for Afghanistan's future.

Rabbani arrived in a jeep with blackened windows, part of a convoy of 15 vehicles accompanied by heavily armed guards. Representatives of other factions which make up the Northern Alliance were with him in the convoy. Four days before, troops from Rabbani's Jamiat-e-Islami, the largest faction in the Northern Alliance, rolled into Kabul following the departure of the Taliban.

Also expected in Kabul on Saturday was United Nations envoy Francesc Vendrell, who was to help work out a plan for a new Afghan government.

Though Rabbani has never relinquished his claim to the presidency, he has acknowledged the international calls for a broad-based government that would include all of Afghanistan's ethnic groups. If Rabbani or any other were to claim power, it could unleash the kind of faction fighting that destroyed much of the city during Rabbani's tenure from 1992-96.

There already are signs of tensions among the factions.

Karim Khalili, who heads the Hezb-e-Wahadat factions representing Shiite Muslims, assailed Rabbani for sending his soldiers to take control of Kabul.

Rabbani says the troops are in Kabul to provide security, though they have taken over the key ministries of defense, foreign and interior.

"The United Nations should move quickly to do something, because it is unfair for one party to be in control," Khalili said by satellite telephone from Jalraiz, 25 miles southwest of the capital.

Khalili said he has brought some 3,000 Shiite Muslims to Jalraiz, and said his followers had put up posters of him throughout the western neighborhoods of the capital, which is dominated by Shiite Muslims.

The Associated Press contributed to this report.