Advanced weapons-tracking radar systems have been deployed by South Korea on islands near its Yellow Sea border, following North Korean artillery shelling last week.
South Korean Defence Minister Kim Tae-young said the AN/TPQ-36 Firefinder systems had been set up on Baengnyeong and Yeonpyeong islands.
The Firefinder system is designed to detect the origin of incoming artillery and rocket fire and allows a possible counterattack.
Last week, North Korea fired 370 shells into the sea near the border over three straight days after declaring two "no sail" zones, a move that has raised concerns over further missile tests on the Korean peninsula.
According to South Korean military, the North had designated two more "firing zones" in the area, effective for four days from 5 February, for possible heavy artillery fire.