"This threat also may exist for places where Americans are known to congregate or visit, including clubs, restaurants, places of worship, schools or outdoor recreation events," the U.S. embassy said in a notice on its Web site (www.beijing.usembassy.gov).

U.S. citizens should be alert to possible threats, the notice said.

Chinese authorities were not immediately available for comment.

The warning gave no further details, but it comes two days after the killing in Iraq of al Qaeda militant Abu Musab al-Zarqawi in a U.S. airstrike in Iraq.

The embassy issued a similar notice in China in November, warning at that time that police had told them Islamic extremists could be planning attacks on luxury hotels in China.