North Korea on Saturday fired two short-range ballistic missiles into the Sea of Japan, South Korea's military said, with the apparent weapons test — its fifth in 16 days — flying in the face of U.S. President Donald Trump's jubilant exhortations after receiving another "beautiful letter" from Pyongyang hours earlier.

The missiles, which were fired from near the northeastern city of Hamhung at 5:32 a.m. and 5:50 a.m., flew 400 km at a peak altitude of 48 km, the South Korean military said, adding that there was a high chance of additional launches coming amid the North's own summer military drills and joint exercises between the U.S. and South.

The South's presidential Blue House said the weapons were believed to be short-range ballistic missiles, raising the prospect that they were similar to recent tests of its KN-23 missile, which appears to be built to avoid detection.