Turkish police have detained four suspected members of the Islamic State group as part of an investigation into a possible attack on the U.S. Embassy in Ankara, Turkey's state-run media said March 5 .

The U.S. Embassy has announced on its website that it would be closed on March 5 because of an an unspecified security threat.

Security was heightened outside the U.S. Embassy building, and police searched pedestrians before allowing them to enter a street where the main gate is located.

The Anadolu news agency said police detained four suspects on March 5 -- all of them Iraqi nationals. Two of them were detained on a bus at a security check on a highway linking the Black Sea port of Samsun to Ankara. Two others were detained in Samsun.

The Turkish capital saw a wave of attacks in 2015 and 2016 in which hundreds were killed.

Based on reporting by AP and dpa