According to CNN, on Friday the US Mission in Turkey issued a warning to American citizens regarding a potential terrorist threat in various locations throughout the country, including Istanbul, its most populous city. 

The US Mission said it "received credible reports of potential terrorist attacks and kidnappings against US citizens and foreign nationals in Istanbul, including against the US Consulate General, as well as potentially other locations in Turkey," according to a security alert issued Friday.

The alert urged US citizens "to exercise heightened caution in locations where Americans or foreigners may gather, including large office buildings or shopping malls."

This is a developing situation, and it's likely the US State Department will provide additional details as officials continue to respond to the possible threat.