U.S. Defense Secretary Donald Rumsfeld declined to comment directly Monday on a published report claiming the U.S. military missed an opportunity to kill the leader of Afghanistan's Taleban, Mullah Mohamed Omar.

Mr. Rumsfeld said U.S. forces are aggressively striking Taleban and al-Qaida terrorist targets, including the group's leaders as well as their troops, weapons and facilities.

But the U.S. Defense Secretary declined Monday to respond directly when asked about an article in The New Yorker magazine by Pulitzer prize winning investigative reporter Seymour Hersh.

Mr. Hersh wrote that an unmanned Predator reconnaissance plane armed with two anti-tank missiles identified a convoy carrying Taleban leader Mullah Omar as he fled Kabul on the first night of U.S. air strikes. But he reported the military failed to strike immediately because the general in charge of the U.S. Central Command received legal advice recommending against targeting the Taleban leader. The decision reportedly infuriated Defense Secretary Rumsfeld.

Responding to questions Monday, Mr. Rumsfeld declined to comment directly although at one point he suggested there might be some validity to the report, using the phrase "that particular event" to describe the incident. He even told reporters he will likely check into what happened.

But at the same time he suggested targeting an individual was too difficult a challenge for U.S. forces. "It is practically impossible to know with certainty who is on the ground at any given location by name and serial number," the secretary said. "It is possible from time to time to see what look to be military leadership elements moving, by the size of the group, by the kinds of vehicles, by the way they conduct themselves, you can make an educated guess that is very likely a military leadership and command element."

Pentagon officials meanwhile declined to discuss the reported use of an unmanned Predator reconnaissance plane to carry offensive weapons.

The Air Force conducted what it described as the first-ever firing of a missile from a Predator only this past February. An announcement at the time said the unmanned aircraft, controlled from the ground, fired a Hellfire missile that struck a stationary Army tank at a test facility in the western state of Nevada.

An Air Force spokeswoman said the testing program has not yet been completed.