SEOUL, South Korea (AP) — North Korea fired three short-range ballistic missiles toward its eastern waters in its latest weapons display on Saturday, a day after rival South Korea conducted a rocket launch related to its push to build space-based surveillance to better monitor the North.

Tensions between the rival Koreas rose this week when South Korea accused North Korea of flying five drones across the tense border for the first time in five years and responded by sending its own drones toward the North.

South Korea’s Joint Chiefs of Staff said in a statement it detected the three launches from an inland area south of Pyongyang, the North’s capital, on Saturday morning. It said the three missiles travelled about 350 kilometres (220 miles) before landing in the waters between the Korean Peninsula and Japan. The estimated range suggests the missiles tested could target South Korea.

The Joint Chiefs of Staff called the launches “a grave provocation” that undermines international peace. It said South Korea maintains a readiness to “overwhelmingly” deter any provocation by North Korea.

The U.S. Indo-Pacific Command said the launches highlight “the destabilising impact” of North Korea’s unlawful weapons programs and that the US commitments to the defence of South Korea and Japan “remain ironclad.” Earlier Saturday, Japan’s Defense Ministry also reported suspected ballistic missile firings by North Korea.

South Korea’s military on Monday scrambled warplanes and helicopters, but they failed to shoot down any of the North Korean drones before they flew back home or vanished from South Korean radar. One of the North Korean drones travelled as far as northern Seoul, triggering security jitters among many people in the South.

South Korea still flew three of its surveillance drones across the border on Monday in an unusual tit-for-tat. South Korea on Thursday staged large-scale military drills to simulate shooting down drones.

South Korean President Yoon Suk Yeol has called for boosting his country’s air defence network and vowed to sternly deal with provocations by North Korea.

Since taking office in May, Yoon’s government has expanded regular military drills with the US in the face of increasing North Korean nuclear threats. North Korea has called such drills an invasion rehearsal and argued its recent missile tests were its response. But some experts say North Korea is using the South Korea-US training as a pretext to modernise its arsenal and increase its leverage in future dealings with the US.

Before Saturday’s launches, North Korea had already test-fired more than 70 missiles this year. Many of them were nuclear-capable weapons designed to attack the US mainland and its allies South Korea and Japan.

Later Saturday, senior diplomats from South Korea, Japan and the United States jointly denounced the North’s launches after a phone call. They agreed to reinforce their deterrence against North Korea and work together to achieve the North’s denuclearization, according to the South Korean and Japanese foreign ministries.

On Friday, South Korea test- launched a solid-fueled rocket, a type of a space launch vehicle that it plans to use to put its first spy satellite into orbit in coming years.

Defence officials said it was a follow-up test of the country’s first successful launch of a solid-fuel rocket in March. The unannounced launch triggered a brief public scare of a UFO appearance or a North Korean missile.

North Korea is also pushing to acquire its first military surveillance satellite. Earlier this month, it said it used two old missiles as space launch vehicles to test a camera and other systems needed for a spy satellite and later released low-resolution satellite photos showing South Korean cities.

Some South Korean experts said the North Korean satellite imagery was too crude for military reconnaissance purposes and that the North Korean rocket launches were likely a disguised test of missile technology. Infuriated over such an assessment, Kim Yo Jong, the powerful sister of North Korean leader Kim Jong Un, issued crude insults against unidentified South Korean experts. She also dismissed doubts over North Korea’s intercontinental ballistic missile technology and threatened to conduct a full-range ICBM test.

This week, North Korea is under a major ruling party meeting in Pyongyang to review past policies and policy goals for 2023. It’s highly unusual for North Korea to test-launch a missile when it holds a key meeting.

In an indication that the plenary meeting of the Workers’ Party was being wrapped up, the North’s state media reported Saturday that its powerful Politburo decided to complete the draft resolution of the plenary meeting.

Some observers said North Korea will likely publish details of the meeting on Sunday, which would carry Kim Jong Un’s vows to expand his nuclear arsenal and introduce sophisticated weapons in the name of dealing with what he calls US hostility.