South Korea's military is on alert with US forces for signs of North Korea preparing to fire a submarine-launched ballistic missile, or SLBM.
South Korea's presidential office says President Yoon Suk-yeol was informed of such signs in a security-related meeting on a government plane on his way back from Canada.
South Korean media say signs of a missile launch were observed in the port city of Sinpho in North Korea's eastern province of South Hamgyong.
It is unusual for the South Korean government to announce signs of a North Korean missile launch.
A missile believed to be an SLBM was fired from Sinpho in May.
The US research group 38 North said on Wednesday its analysis of satellite images of a shipyard in Sinpho indicates the North appears to be preparing to tow out a new submarine that can be loaded with an SLBM.
The US nuclear-powered aircraft carrier Ronald Reagan docked in the southeastern South Korean city of Busan on Friday ahead of joint military drills between South Korea and the United States around the Sea of Japan.
The two countries are stepping up vigilance as the North may react negatively to the drills.
Japanese Defense Ministry officials say North Korea may have launched a ballistic missile on Sunday.
The officials tell NHK the projectile may have landed outside Japan's Exclusive Economic Zone.