"The Department of State made this decision based on security information indicating extremist groups are continuing aggressive efforts to attack US citizens in areas of Istanbul where they reside or frequent," the warning said.
The warning goes beyond the one issued Monday that urged Americans to avoid traveling to southeast Turkey, especially urban centers near the Turkish-Syrian border, because of recent terrorist attacks.
Turkey has become increasingly volatile in the past year because of terrorist attacks at home and its involvement in the Syrian civil war.
