The United States and Israel on Saturday warned their citizens of a high-level, imminent threat of attacks in Turkey, with Israel urging its citizens to immediately leave the country.

Turkey has been rocked by four suicide bombings already this year, the most recent last month in Istanbul. Two of those have been blamed on Islamic State of Iraq and the Levant (ISIL), while Kurdish fighters have claimed responsibility for the other two.

The US embassy emailed what it called an "emergency message" to Americans, warning of "credible threats" to tourist areas in Istanbul and the resort city of Antalya. Israel announced "immediate risks".

"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," it said.

Later on Saturday, three people were slightly wounded after a small bomb left on the side of a road exploded in Istanbul's central Mecidiyekoy district, Turkey's state-run Anadolu news agency reported.

The blast came from a non-lethal stun grenade, designed to create a loud noise and blinding flash.

The three victims were taken to the hospital.

Earlier on Saturday, two Reuters news agency reporters in central Istanbul saw an extremely heavy police presence with roads sealed off. Armed special police units were deployed outside foreign consulates, including the German and Italian missions.

Al Jazeera's Harry Fawcett, reporting from Istanbul, said the attacks and security alerts are keeping people on edge in Turkey's largest city.

"There is heightened security, as there has been for many weeks now," Fawcett said.

"There is a real sense of concern," he added. "There is no panic, people are still going about their daily lives, but the amount of foot traffic in places that would normally be much more congested is down.

"People are thinking twice about plans in terms of socialising, where to go and whether it makes sense to take certain risks."

'Immediate risks'

Tensions between the Turkish government and Kurdish separatists have boiled over, and strikes by ISIL have intensified over the past year.

Three different attacks in the capital Ankara during the past six months killed more than 170 people.

Israel's counterterrorism bureau said on Saturday that it was reiterating and sharpening the high level of threat in Turkey following a situational assessment.

"There are immediate risks of attacks being carried out in the country, and we stress the threat applies to all tourism sites in Turkey," the bureau said.