Although the Bush administration is moving to shut down a pipeline that provided Iraqi oil to Syria illegally, the United States and American oil companies have long imported Syrian oil, more of which became available after the pipeline opened about three years ago, oil industry experts here said today.

Industry experts and oil traders said that about a week ago Syria told buyers of its oil that exports would decrease by 200,000 barrels a day for the rest of the year, indicating that it knew the pipeline would be closed or that it had been closed already.

Those experts and United Nations diplomats said that the pipeline, which carried 150,000 to 250,000 barrels a day, was an open secret in the industry and the United Nations Security Council. Council members' attempts to get an explanation from Syria were blocked by Russia, France and China, a United Nations diplomat said, and by Syria itself when it became a Council member.

Both Syria and Iraq benefited enormously from the pipeline, said Fadhil J. Chalabi, a former acting secretary general of OPEC from Iraq and now executive director of the Center for Global Energy Studies in London, a research and consulting concern. ''Saddam got cash outside the United Nations fund,'' he said.