A North Korean soldier defected across the demilitarized zone that divided the two Koreas on Monday afternoon, in a rare incident at the truce village of Panmunjeom, according to South Korea's Joint Chiefs of Staff.
South Korea has said the joint drill, due to finish on Tuesday, was in response to North Korean nuclear and missile provocations and to show any such developments by Pyongyang can be repelled with "overwhelming force".
File image of a North Korean soldier, centre top, looks at the southern side as South Korean soldiers stand guard at the border village of Panmunjom. South Korea was keeping its "full combat readiness against North Korea's possibility provocations", its military said Monday.
However, Pyongyangs state news agency accused the US and South Korea of attempting to escalate tensions in the region.
"(He) then exited the vehicle and continued fleeing south" across the military demarcation line after being shot by North Korean troops, according to the United Nations Command (UNC).
Since its last ballistic missile launch September 15, North Korea has not conducted any provocations, raising hopes for possible talks on the long-standing North Korean nuclear stalemate. "And do not try us. This would be a fatal miscalculation". They are putting your regime in grave danger.
The drill involves 21 vessels - including 14 from the United States and seven from South Korea, with the latter equipped with the Aegis Combat System which allows them to track and guide weapons to destroy enemy targets.
About 30,000 North Koreans have defected to South Korea since the end of the 1950-53 Korean War, but most travel through China.
While in South Korea last week, Trump offered North Korea an opportunity to embrace diplomacy instead of continuing its pursuit of nuclear weapons capable of reaching the mainland U.S.
The carriers are required to meet up in the Sea of Japan during the week off the Korean Peninsula, according to a U.S. defense official.