Afghanistan, April 2 -- The U.S. Embassy in Baghdad has urged American citizens to leave Iraq immediately, warning of possible attacks in the coming days amid rising fears of militia activity and broader regional instability.

In a new security alert, the embassy said Iran-aligned armed groups may be planning attacks in central Baghdad within the next 24 to 48 hours. The warning said potential targets could include U.S. citizens, businesses, universities, diplomatic facilities, hotels, airports, energy infrastructure and other locations seen as linked to the United States.

The embassy also warned that some attackers may appear to be connected to Iraqi state institutions or carry official identification, increasing concern over the complexi...