(CNN) -- U.S. officials say they have "credible reports" that Mohammed Atef -- one of the highest-ranking members of al Qaeda, has been killed, and the secretary of defense revealed Friday that U.S. Special Forces are fighting in Afghanistan.

Although officials say the intelligence information about Atef appears credible, "You can't say he's definitely dead until we see the body," said one official.

Atef's daughter is married to one of Osama bin Laden's sons and he is frequently spoken of as a possible successor to bin Laden. The officials did not say when the attack happened.

Atef is believed to be responsible for supervising the training of operatives. Prosecutors say he provided military training and assistance in 1993 to Somali tribes who violently opposed the United Nations' intervention in Somalia's civil unrest. In an October 1993 battle, Somali tribesmen killed 18 U.S. Army Rangers.

Meanwhile, U.S. Special Forces are taking part in ground combat in Afghanistan, Defense Secretary Donald Rumsfeld said Friday. He said no Americans have been killed.

In northern Afghanistan thousands of Taliban soldiers, armed with an estimated 100 artillery pieces and 60 tanks, were fighting the Northern Alliance for control of Konduz. (Full story)