Al Qaeda's No. 2, Ayman Al-Zawahiri, said in a videotape aired Friday that the United States' recent decision to withdraw some troops from Iraq represented "the victory of Islam."

Al-Zawahiri , wearing a white turban and gray robe and seated next to an automatic rifle, waved his finger for emphasis as he spoke in the two-minute excerpt aired by Al-Jazeera.

"I congratulate [the Islamic nation] for the victory of Islam in Iraq," he said.

Al-Zawahiri apparently was referring to comments last month by Defense Secretary Donald H. Rumsfeld, who said President Bush had authorized new troop cuts below the 138,000 level that prevailed for most of last year.

Rumsfeld did not reveal the exact size of the cut, but the Pentagon said the reductions would be about 7,000 troops, about the size of two combat brigades. The Pentagon has not announced a timetable for troop reductions, but indications are that the force could be cut significantly by the end of this year.

"You remember I told you more than a year ago that the American withdrawal from Iraq is only a matter of time, and here they are now ... negotiating with the mujahedeen," Al-Zawahiri said.

"Bush was forced at the end of last year to announce that he will pull out his forces from Iraq, but he was giving excuses for his withdrawal that the Iraqi forces have reached a good level."

Bush has not offered concrete details about bringing troops home but said Wednesday that "possible adjustments" would be discussed with Iraqi leaders if progress continued on security and political efforts.

A U.S. counterterrorism official, speaking on condition of anonymity in compliance with office policy, said that while the video appears to be legitimate, it has not been determined with certainty that it is Al-Zawahiri .

However, officials often note that no message has ever been fraudulently attributed to Al Qaeda's leadership.

Al-Zawahiri said the American forces "with their planes, missiles, tanks and fleets are mourning and bleeding, seeking for a getaway from Iraq."

"Regarding your withdrawal timetable ... you have to admit, Bush, that you have been defeated in Iraq and are being defeated in Afghanistan and will be defeated in Palestine," he said, speaking calmly but forcefully.

Al-Jazeera said the videotape was dated in December but it gave no more specifics. Network spokesman Ayman Gaballah said the videotape, about 20 minutes long, was acquired through "a special source." He would not elaborate.

More excerpts were expected to be aired later Friday.

The most recent videotape from Al-Zawahiri , aired by Al-Jazeera in October, called on Muslims to aid victims of a massive earthquake in Pakistan.

"If I was comforting the Islam nation for its crisis of Pakistan's earthquake, I today congratulate it and bless the Islamic victory in Iraq," Al-Zawahiri said in Friday's tape.