The exact nature of the threat was unclear. A spokesman for the U.S. Embassy in Ankara, the Turkish capital, said the notice was issued “as a result of our ongoing assessment of security conditions.”
A Turkish government spokesman said the country’s Interior Ministry would release a statement about the security warning later Friday.
Istanbul, Turkey’s largest city and its commercial center, suffered a rash of terrorist attacks a few years ago, including several that authorities said were carried out by the Islamic State militant group.
Turkish authorities in recent weeks have arrested dozens of people they said were affiliated with the Islamic State, including Mahmut Ozden, the militant group’s senior leader in Turkey, according to the Interior Ministry.