Intelligence officials who have seen the evidence consider it "extremely compelling," the U.S. official said, adding that it was gleaned from a variety of sources, not just Israeli intelligence.

The official spoke on condition of anonymity late Wednesday because of the sensitive nature of the information.

CIA Director Michael Hayden and other intelligence officials are to brief Congress on the evidence related to the bombed Syrian facility in appearances Thursday before six committees, including the Senate and House intelligence committees.

The Syrian reactor was similar in design to a North Korean reactor that has in the past produced small amounts of plutonium, the official said. It was not yet complete but was far enough along to demonstrate a resemblance to the North Korean reactor at Yonbyon.

The official said no uranium — the fuel for a reactor — was evident on site. Syria has maintained in the past that the site was an unused military facility.

Syria did not declare the apparent reactor to the International Atomic Energy Agency nor was it under international safeguards, possibly putting Syria in breech of an international nuclear nonproliferation treaty.

Plutonium-producing reactors are of international interest because plutonium can be used to make high-yield nuclear weapons.

Israeli warplanes bombed a site in Syria on Sept. 6, 2007, that private analysts say appears to have been the site of a reactor, based on commercial satellite imagery taken after the raid. The site later was razed.

U.S. officials said Israel shared intelligence with the United States before the bombing after administration officials expressed doubts that the site was a nuclear reactor built with North Korea's assistance, according to The Washington Post, which first reported the existence of the videotape on its Web site Wednesday.

The target of Israel's raid has been veiled in secrecy, with U.S. intelligence and government officials refusing to confirm until now that suspicions that the site was to be a nuclear reactor.

The revelation of alleged North Korean cooperation with Syria comes at a sensitive time for Pyongyang.

U.S. diplomats are pressing North Korea to come clean about its nuclear cooperation with Syria as part of those talks but have had little success.

Under an agreement reached last year with the United States, China, Japan, South Korea and Russia, the North is required to give a full account of its nuclear programs, including whether it spread nuclear technology.

North Korea claims it gave the nuclear declaration to the U.S. in November, but U.S. officials say the North never produced a "complete and correct" declaration.

The Capitol Hill briefings come the same week a U.S. delegation went to North Korea to press the regime for a detailed list of its nuclear programs, the latest sticking point at international nuclear disarmament talks.