North Korea on Saturday fired two short-range ballistic missiles toward its eastern waters, South Korean and Japanese officials said, making it the North’s fourth round of weapons launches this week that are seen as a response to military drills among its rivals.

South Korea’s military said that it detected the two North Korean missile launches 18 minutes apart on Saturday morning coming from the North’s capital region.

Japan’s Defense Ministry said it also spotted the launches, AP reported.

“The repeated ballistic missile firings by North Korea are a grave provocation that undermines peace and security on the Korean Peninsula and in the international community,” South Korea’s Joint Chiefs of Staff said in a statement.

It said South Korea strongly condemns the launches and urges North Korea to stop testing ballistic missiles.

Toshiro Ino, Japan’s vice defense minister, called the launches “absolutely impermissible.” He said the four rounds of missile testing by North Korea in a week is “unprecedented.” According to South Korean and Japanese estimates, the North Korean missiles flew about 350-400 kilometers (220-250 miles) at a maximum altitude of 30-50 kilometers (20-30 miles) before they landed in the waters between the Korean Peninsula and Japan.

Ino, the Japanese vice minister, said the missiles showed “irregular” trajectory.

The five other ballistic missiles fired by North Korea on three occasions this week also show similar low trajectories.

Saturday’s launches came a day after South Korea, Japan and the United States held their first trilateral anti-submarine drills in five years off the Korean Peninsula’s east coast.

Earlier this week, South Korean and US warships conducted bilateral exercises in the area for four days.

Both military drills this week involved the nuclear-powered aircraft carrier USS Ronald Reagan and its battle group.