ISTANBUL — The State Department issued an unusually sharp warning Friday about new threats to U.S. citizens in Istanbul, saying that the U.S. Mission in Turkey has received "credible reports of potential terrorist attacks and kidnappings" targeting Americans and other foreign nationals.

A brief statement also referred to unspecified threats against the U.S. Consulate in Istanbul and said services there and at other U.S. missions around Turkey would be suspended. It advised U.S. citizens to “exercise heightened caution in locations where Americans or foreigners may gather, including large buildings or shopping malls.”

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.

In the last major incident, an attack in 2017 that was claimed by the Islamic State, a gunman killed dozens of New Year’s revelers at Istanbul’s waterside Reina night club. The gunman, Abdulkadir Masharipov, an Uzbek national, was sentenced in September to life in prison without the possibility of parole.

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.