Quoting the US Consulate General security message:

"The Consulate General advises U.S. citizens residing in or visiting Istanbul to review and update their personal security practices when traveling to locations where westerners are known to frequent or reside. U.S. citizens should also exercise increased vigilance and caution in areas easily accessible to the general public."

Recommendations:

• Avoid travel to southeastern Turkey especially large urban centers near the Syrian border.
• Avoid crowds, including at popular tourist destinations.
• Be vigilant and cautious when visiting public areas especially those frequented by tourists.
• Avoid political gatherings and rallies.
• In an emergency, obey the instructions of local authorities.
• Stay at hotels with identifiable security measures in place.
• Monitor local media.