The move was the latest step in the fallout over a U.S. air attack in northern Syria last Sunday in which eight Syrians died. U.S. military officials said the fight took place as soldiers looked for smugglers of fighter planes into Iraq.

DEBKAfile reported that military sources said the Syrian government has warned that if there are more U.S. raids Syria will break off security cooperation not only with the United States but also with Iraq on their common border.

Reports say the situation is explosive enough to lead to a Syrian declaration of war if Iraqi forces hit terrorist bases on its soil.

A U.S. embassy spokesman said concern was high about the continued threat of "terrorist attacks, demonstrations and other violent actions against U.S. citizens."

Sources said the embassy had prepared for "extreme events," such as a Syrian military siege, violent demonstrations or a direct attack that would force an evacuation of U.S. personnel.