The U.S. aircraft carrier USS George Washington and at least 10 other U.S. ships joined forces Sunday with the South Korean navy in the Yellow Sea near the border of the two Koreas, The New York Times reported.
The exercises had been announced earlier, but took on more significance after North Korea shelled a South Korean military base on the island of Yeonpyeong Tuesday and killed four people, two of them civilians.
"Monday's drills include a live-fire exercise by multiple aircraft from the George Washington, which will shoot mock targets in waters," an official from South Korea's Joint Chiefs of Staff told Yonhap on condition of anonymity.
The U.S. E-8C Joint Surveillance Target Attack Radar System is also at work closely monitoring North Korea's military moves, Yonhap said.
Since tensions heightened, the scale of the exercises was increased, a military official told the Times.
"The intensity for the Yellow Sea drills will be higher than planned," the official said. "Participating troops will conduct live-fire shooting and bombing drills."
South Korea's Defense Ministry Sunday ordered civilians and about 400 journalists to leave the island by the end of the day for their own safety, based on an incident Friday when North Korea filed a single shell at the island after a visit by a U.S. general, reports said.
Meanwhile, South Korea acknowledged it had mistakenly fired a howitzer toward the demilitarized zone Sunday, Yonhap reported.
The 155mm shell fell in South Korea, 10 miles south of the border. Seoul notified Pyongyang of the accident, saying it was a training mishap, the report said.