The United States embassy in Turkey on Saturday warned American citizens of "credible threats" to tourist areas in Istanbul and the resort city of Antalya.

The United States embassy in Turkey on Saturday warned American citizens of "credible threats" to tourist areas in Istanbul and the resort city of Antalya.

The heightened alerts come three weeks after a suicide bomber struck a popular shopping street in the heart of Istanbul -- killing four people and injuring dozens -- in an attack which authorities blamed on the ISIL group.

"The U.S. Mission in Turkey would like to inform U.S. citizens that there are credible threats to tourist areas, in particular to public squares and docks in Istanbul and Antalya," read an emergency travel warning published on the U.S. embassy's official website.

"Please exercise extreme caution if you are in the vicinity of such areas."