South Korea fired on a North Korean patrol boat Monday, just a day after the North drew international condemnation by testing another banned long-range missile.

According to a report by South Korea’s Yonhap News Agency, the incident came as the Northern craft “crossed the de facto western maritime border between the two Koreas.”

Citing “a South Korean military official,” Yonhap said the boat crossed the Northern Limit Line in the Yellow Sea about 6:55 a.m. Monday local time and stayed on the southern side of the line, which North Korea doesn’t acknowledge as legitimate, for about 20 minutes before retreating under five rounds of South Korean fire.

“The South Korean military is on high alert, beefing up surveillance near the NLL and monitoring any abnormal activities by North Korean soldiers,” the official told Yonhap.

On Sunday, North Korea test-fired a long-range missile, which it said was intended to put a satellite into orbit, but which South Korea, Japan and the U.S. condemned as a test for a nuclear weapon delivery system.

When tensions are high between the two Koreas, which are still technically at war, their respective militaries often fire on each other or engage in provocative maneuvers or posturing.