WASHINGTON (AP) _ President Clinton signed an executive order imposing a U.S. trade embargo on the Yugoslav republic of Serbia, a White House official said today.

The main thrust of the order was to begin cutting off the supply of oil to Milosevic's forces, National Security Council spokesman David Leavy said.

``As we continue to intensify the air campaign, this is another step in tightening the noose around President Milosevic's war machine,'' Leavy said. ``The United States will continue to tighten the screws until our objectives are met.