Gordon D. Johndroe, a White House spokesman, declined to comment on the satellite pictures.

A reactor of the size of what analysts believe Syria was building would have been able to make enough plutonium to fuel about one nuclear weapon a year. But removing the plutonium from spent fuel rods would require a reprocessing facility for which analysts have reported no evidence.

Satellite images of the Syrian site were released by DigitalGlobe, in Longmont, Colo., and SPOT Image Corporation, in Chantilly, Va. They show a smooth, unfurrowed area where the large building once stood.

“It’s clearly very suspicious,” said Joseph Cirincione, an expert on nuclear proliferation at the Center for American Progress in Washington. “The Syrians were up to something that they clearly didn’t want the world to know about.”

Mr. Cirincione said the photographic evidence “tilts toward a nuclear program,” but did not prove that Damascus was building a reactor. Besides, he said, even if Syria was developing a nuclear program, it was still years away from being operational and thus not an imminent threat.

The desolate Syrian site is situated on the eastern bank of the Euphrates River some 90 miles north of the Iraqi border and seven miles north of the desert village of At Tibnah. An airfield lies nearby.

The new images, in addition to revealing the removal of the tall building, show still standing a secondary structure and what could be a pumping station on the Euphrates. Analysts suspect the pumping station was for cooling the reactor.

The building was said by analysts to have been modeled on a design used by the North Koreans, whose building is a few feet larger that the Syrian building that vanished.