Nuclear-armed North Korea fired off two ballistic missiles into the Sea of Japan on Wednesday evening, Japan¡¯s Defense Ministry said, just a day before U.S. Vice President Kamala Harris was due to visit South Korea.

It was the isolated country¡¯s second missile launch in four days.

Japanese State Minister for Defense Toshiro Ino told reporters that the missiles were believed to have landed in waters outside Japan¡¯s exclusive economic zone, which extends 200 nautical miles (370 kilometers) from its coast.

Ino said the first missile had flown about 350 km, hitting a maximum altitude of around 50 km, while the second had traveled 300 km and also hit an altitude of 50 km, and that they appeared to fly on irregular trajectories.

South Korea¡¯s military also confirmed the launches, classifying the weapons as short-range ballistic missiles and saying that they had been launched from the vicinity of Pyongyang and flew some 360 km, hitting top speeds of about Mach 6 while reaching maximum altitudes of around 30 km.

Observers said the weapons could be the North¡¯s KN-23 missiles, which are able to perform ¡°pull-up¡± maneuvers that help them better evade defenses. South Korea¡¯s Yonhap news agency, quoting unidentified sources, said the weapons may have been fired from mobile launchers.

On Sunday, the North launched what appeared to be a short-range ballistic missile for its first major weapons test since early June. Pyongyang typically releases details of its weapons tests the following day, but on Monday there were no reports of the launch.

Harris was set to visit Seoul from Thursday, after traveling to Japan for the state funeral of former Prime Minister Shinzo Abe. In South Korea, the U.S. vice president is scheduled to visit the Demilitarized Zone that separates the two Koreas.

Observers say the launches could be a response to ongoing joint U.S. military drills with South Korea involving the USS Ronald Reagan aircraft carrier strike group ¡ª based in Yokosuka, Kanagawa Prefecture ¡ª in the waters near the South.

Earlier Wednesday, South Korea¡¯s spy agency said that the North has readied an underground tunnel at its Punggye-ri nuclear test site. A blast could take place sometime between Oct. 16 and Nov. 7, Yonhap reported. Such a time frame would position any test between a rare congress of China¡¯s Communist Party and November¡¯s U.S. midterm congressional elections.

Last week, media reports said that South Korea¡¯s military had detected signs that the North could also be preparing to test a submarine-launched ballistic missile, after an analysis of satellite imagery spotted unusual movements in the port city of Sinpo.