South Korea fired warning shots at a North Korean patrol boat that crossed the maritime border in the West Sea on Saturday, amid escalating tension prompted by the North’s latest missile test Thursday.

South Korea’s Joint Chiefs of Staff said Sunday that it fired the shots after the North had not responded to warnings, pushing the boat back to the North from the de facto sea boundary known as the Northern Limit Line.

The NLL, disputed by Pyongyang since 1953 when it was drawn up following the 1950-53 Korean War, saw naval skirmishes that had led to the deaths of 55 South Korean service members from 2002 to 2010. Since 2016, Seoul has annually held a memorial service in late March to honor their sacrifices.

“Our military maintains unmistakable readiness, closely monitoring North Korea’s movements and reading ourselves for its potential provocations,” the JCS said in a statement, noting poor visibility had led to a minor collision between a South Korean patrol ship and a nearby Chinese fishing vessel that the North Korean boat was chasing. The Chinese vessel had first crossed the border.

An inter-Korean confrontation around the NLL last took place in October, when the two Koreas traded warning shots, accusing each other of breaching the border. The South Korean Navy fired 20 artillery shells to see off a North Korean commercial vessel.

Meanwhile, efforts to dial back North Korea’s aggression are seeing little headway. The isolated country facing international sanctions over its nuclear and missile programs on Thursday tested a solid-fuel intercontinental ballistic missile -- an advance in technology that experts say would make the weapon faster to launch and harder to detect.

South Korea and the US, its biggest ally, are stepping up readiness, having resumed their annual March field maneuvers after a five-year hiatus yielded no progress on denuclearization. Drills involving the joint air forces and marines will take place from Monday to April 28, according to South Korea’s Air Force.

Pyongyang labels such drills as “rehearsals for war,” and has threatened to use nuclear weapons if Seoul and Washington do not suspend what they call “tests for readiness.”

The United Nations Security Council will convene a meeting Monday to discuss North Korea’s latest weapons test. But endorsing resolutions or UNSC presidential statements, a more symbolic gesture meant to rally international opinion, are widely not expected to follow, given the veto powers of China and Russia, Pyongyang’s biggest supporters.