"The US government continues to receive credible indications that terrorist groups are seeking opportunities to attack popular tourist destinations throughout Turkey," the embassy in Ankara said in a statement emailed to U.S. citizens.
"Foreign tourists in Turkey have been explicitly targeted by terrorist organizations," the U.S. embassy said in what it described as an "emergency message".
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.
