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."
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.
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It called on all Israelis to avoid visiting Turkey and urged Israeli tourists there to leave "as soon as possible", defining the threat as level 2 - "concrete and high". The country made a similar warning on March 28.
Three Israelis and an Iranian were killed and 39 people wounded when a man blew himself up on Istiklal Street, a famous shopping thoroughfare in the heart of Istanbul, on March 19.
Turkey said the bomber had links to ISIL, also known as ISIS. On Tuesday, Israel's defence ministry said he had most probably targeted the Israelis deliberately.
Another attack in Istanbul's old city, the Sultanahmet district, targeting tourists killed 12 Germans and one Peruvian in January.
Tens of thousands of Israelis visit nearby Turkey each year despite strained diplomatic relations between the two countries.
On March 29, President Reuven Rivlin expressed deep concern over reports that ISIL was planning to attack Jewish schoolchildren in Turkey.
A NATO member country, Turkey was a regional ally of Israel until the two cut ties in 2010 over the deadly storming by Israeli commandos of a Turkish aid ship bound for the Gaza Strip, which left 10 Turkish activists dead.
The two sides have been in talks to repair the damaged links.
On March 29, the US State Department and Pentagon ordered the families of American diplomats and military staff to leave posts in southern Turkey because of security fears.
Turkey is facing multiple security threats. As part of a US-led coalition, it is fighting ISIL in neighbouring Syria and Iraq. It is also battling Kurdish fighters in its southeast, where a ceasefire collapsed last July, triggering the worst violence since the 1990s.
| Turkey's capital Ankara rocked by deadly explosion
Source: Al Jazeera and agencies