The United States warned its citizens on Saturday about "credible threats" to tourist areas in Turkey, particularly in Istanbul and the southwest coastal resort of Antalya.
Turkey has been hit by four suicide bombings already this year, the most recent one last month in Istanbul. Two of those have been blamed on Islamic State, while Kurdish militants have claimed responsibility for the other two.
Click through images from a suicide bomb explosion in Istanbul:
The U.S. Embassy emailed what it called an "emergency message" to Americans.
"The U.S. Mission in Turkey would like to inform U.S. citizens that there are credible threats to tourist areas, in particular to public squares and docks in Istanbul and Antalya," it said in the statement. "Please exercise extreme caution if you are in the vicinity of such areas."
Two Reuters reporters in central Istanbul saw an extremely heavy police presence, with roads sealed off near the Hilton hotel. Armed special police units were deployed outside foreign consulates in Istanbul, including the German and Italian missions.
Last month's attack in Istanbul's main shopping district killed three Israelis, two of whom held dual citizenship with the United States, and one Iranian. A separate attack in the city's historic heart in January killed 12 German tourists.
Turkey is facing multiple security threats. As part of a U.S.-led coalition, it is fighting Islamic State in neighboring Syria and Iraq. It is also battling Kurdish militants in its southeast, where a 2-1/2-year ceasefire collapsed last July, triggering the worst violence since the 1990s. (Reporting by Ayla Jean Yackley and Nick Tattersall; Writing by David Dolan; Editing by Mark Heinrich)