U.S. Secretary of State John Kerry and his South Korean counterpart are warning North Korea against any possible aggression amid mixed signals from the North over returning to denuclearization talks and strains between South Korea and Japan that threaten coordination on the issue.

Kerry and Foreign Minister Yun Byung-sen on Thursday dismissed demands from the North to halt an upcoming joint U.S.-South Korean military exercise. They said the exercise could not be used as an excuse by North Korea to stay away from talks or to delay attempts to improve relations between the North and South.

Kerry said the United States is firmly committed to South Korea's defense. He added that it is vitally important for Japan and South Korea to overcome historical animosities to present a united front.