The United States warned its citizens today 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. The attacks have been blamed on the Islamic State militant group.

In what it called an "emergency message", the U.S. Embassy in Turkey warned American citizens to exercise extreme caution.

"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 the statement emailed to U.S. citizens in Turkey.

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 neighbouring 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.