U.S. and South Korean forces launched maritime exercises in the Yellow Sea this week. Similar maneuvers this year sparked already heightened tensions on the Korean Peninsula.
North Korean rhetoric that followed its February test of a nuclear device raised concerns about regional security. South Korean news agency Yonhap this week said tensions were easing, though China's official Xinhua News Agency reported the North Korean government vowed to "mercilessly avenge" any violations during Yellow Sea drills.
Xinhua quoted the North Korean military as saying it would turn all five islands in the Yellow Sea into a "sea of flames" if provoked.
The maneuvers come as South Korea President Park Geun-hye was in the United States, where she met with U.S. President Barack Obama and was to address a joint session of Congress.
Obama said during a Tuesday statement alongside Park that North Korea wasn't gaining anything with its aggressive policies.
"The days when North Korea could create a crisis and elicit concessions are over," he said.