The United States embassy in Turkey on Saturday 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 authorities blamed on the Islamic State 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 in the capital Ankara and Istanbul -- blamed on Kurdish militants or jihadists -- have put the country on high alert and seen foreign embassies put its 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," Israel's counter terrorism bureau said.

"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.

However the series of suicide attacks as well as a spat with Russia have hit the tourism sector hard.

Russia ordered its travel agencies to stop selling trips to the country, a major destination for Russians, after Turkey shot down one of its jets on the border with Syria.

The tourism ministry has reported visitor numbers plunged 10 percent in February, and that was before the latest Istanbul attack and a car bomb which killed 35 people in Ankara.

Local media reported this week that Turkey was planning to increase flights to Iran in a bid to woo visitors elsewhere.

The state-run Anatolia news agency reported Turkish tour operators will focus their attention on Ukraine instead of Russia, with Turkey hoping to attract one million visitors from its Black Sea neighbour to compensate for losses elsewhere. 

Tourism is a mainstay of the Turkish economy and the blow to the sector is especially hard ahead of the crucial summer season.

According to official statistics, foreign tourism brought in almost $31.5 billion in revenues in 2015.