North Korea fired multiple ballistic missiles toward the sea off its east coast on Monday, South Korea’s military said, marking Pyongyang’s fourth launch in a month and following Seoul’s recent test of a hypersonic missile.

The ballistic missiles, which appeared to be of different types, were fired at around 9:20am local time (0020 GMT) from near Pyongyang, and one type flew more than 900 km (559 miles), South Korea’s Joint Chiefs of Staff said.

Earlier, Japan’s coast guard said North Korea had fired two unidentified projectiles, believed to be ballistic missiles, adding that both appeared to have already fallen.

South Korea’s National Security Office convened an emergency meeting, where it condemned the missile launches as violations of United Nations Security Council resolutions and urged an immediate stop.

North Korea has been under UN sanctions since 2006, aimed at halting Pyongyang’s development of nuclear weapons and ballistic missiles.

The latest launches come at a time when tensions have been rising between the two Koreas in recent weeks amid weapons tests by both countries.

Last week, South Korean President Lee Jae Myung oversaw the country’s first test-firing of a hypersonic missile, a rare public showcasing of a missile test by a South Korean president.

Monday’s missile launches by North Korea appeared to be “a show of force in response to the successful test launch of our hypersonic missile, as well as an intentional demonstration of missile capabilities,” a spokesperson for South Korea’s Joint Chiefs of Staff said.

North Korea last fired a ballistic missile on October 3, with state media KCNA saying it was a hypersonic missile tested under the supervision of leader Kim Jong Un.

The tests followed a border mine blast last month that wounded three South Korean soldiers. Seoul said the landmines had been planted by North Korea and demanded an apology, but Pyongyang has dismissed this and accused South Korea of provocation.

Pyongyang has conducted missile tests on more than 10 occasions this year, including four in the last month.

The unusually frequent tests indicate Pyongyang wanted to increase its leverage ahead of a potential summit between leader Kim and US President Donald Trump, said Song Seong-jong, a professor at Daejeon University in Seoul and a former South Korean Defence Ministry official.

“They definitely need to raise the value of their status before meeting Trump,” Song said.