Broadcaster 'SBS' reported that the defence ministry was considering deploying a maritime patrol aircraft, a naval logistics support ship and an explosive ordnance disposal (EOD) team.
Seoul's defence ministry said multiple Russian aircraft had entered and then left its air defence identification zone over the East Sea, also called the Sea of Japan.
U.S. President Donald Trump had earlier abruptly ordered the Pentagon to “substantially reduce” the Ulchi Freedom Shield exercises just before they began Monday.
A turbulent few days have raised doubts about the role of South Korea in any diplomatic push, despite President Lee Jae Myung's dovish line on the North since taking power last year.
