ISTANBUL • The United States Embassy in Turkey yesterday warned American citizens of "credible threats" to tourist areas in Istanbul and the resort city of Antalya, a day after Israel spoke of "imminent risks" of attacks.

The heightened alerts come three weeks after a suicide bomber struck a popular shopping street in the heart of Istanbul - killing four people and injuring dozens - in an attack which the authorities blamed on the Islamic State in Iraq and Syria group.

"The US Mission in Turkey would like to inform US citizens that there are credible threats to tourist areas, in particular to public squares and docks in Istanbul and Antalya," read an emergency travel warning published on the US Embassy's official website. "Please exercise extreme caution if you are in the vicinity of such areas."

A series of attacks - blamed on Kurdish militants or jihadists - in the capital Ankara and Istanbul has put the country on high alert and seen foreign embassies put their residents on guard.

On Friday night, Israel reissued a warning to its citizens to avoid Turkey or "leave as soon as possible". The March 19 Istanbul attack left three Israelis and an Iranian dead.

"Following a situational assessment, we are reiterating and sharpening the high level of threat in Turkey," said Israel's counter-terrorism bureau. "There are immediate risks of attacks being carried out in the country, and we stress the threat applies to all tourism sites in Turkey."

Istanbul - Turkey's biggest city and historic centre which straddles Europe and Asia - and Antalya, with its turquoise Mediterranean waters, are both firm tourist favourites.

AGENCE FRANCE-PRESSE