ISTANBUL—The U.S. embassy in Turkey warned Americans on Monday of possible attacks against churches, synagogues, and diplomatic missions in Istanbul, marking its second such notice in four days, following Koran-burning incidents in Europe.

In an updated security alert, the U.S. embassy said “possible imminent retaliatory attacks by terrorists” could take place in areas frequented by Westerners, especially the city’s Beyoglu, Galata, Taksim, and Istiklal neighborhoods.

Turkish authorities are investigating the matter, it added.

On Friday, several embassies in Ankara including those of the United States, Germany, France, and Italy issued security alerts over possible retaliatory attacks against places of worship, following separate incidents in which the Koran was burned in Sweden, Netherlands, and Denmark.

On Saturday, Turkey warned its citizens against “possible Islamophobic, xenophobic, and racist attacks” in the United States and Europe.