North and South Korean troops exchanged fire along their tense border on Sunday, the South’s military said, the first such incident since the rivals took unprecedented steps to lower front-line animosities in late 2018.

Violent confrontations have occasionally occurred along the border, the world’s most heavily fortified. While Sunday’s incident is a reminder of persistent tensions, it didn’t cause any known casualties on either side and is unlikely to escalate, observers said.

North Korean military guard post in Paju

The Joint Chiefs of Staff in Seoul said in a statement that North Korean troops fired several bullets at a South Korean guard post inside the border zone. South Korea responded with a total of 20 rounds of warning shots on two occasions before issuing a warning broadcast, it said.

South Korea suffered no casualties, the military said. Defense officials said it’s also unlikely that North Korea had any casualties, since the South Korean warning shots were fired at uninhibited North Korean territory. The North’s official Korean Central News Agency, or KCNA, did not immediately report about the incident.

A preliminary South Korean analysis showed that North Korea’s firing was probably not a calculated provocation, though Seoul will continue examining whether there was any motivation for the action, a South Korean defense official said. He spoke on condition of anonymity, citing department rules.

U.S. Secretary of State Mike Pompeo also said it was believed that North Korea’s firing was not intentional.

“We think those are accidental,” Pompeo said on ABC’s “This Week.” “South Koreans did return fire. So far as we can tell, there was no loss of life on either side.”

Farming activities around the North Korean area where the firing occurred continued throughout Sunday and North Korea’s military didn’t display any other suspicious activities after the gunfire, the South Korean defense official said. He said there was thick fog in the area at the time of the incident.

Later Sunday, South Korea sent a message to North Korea to try to avoid an escalation, but the North did not immediately reply, according to South Korea’s military.

The exchange of fire came a day after North Korea broadcast video of its leader, Kim Jong Un, reappearing in public after a 20-day absence amid intense speculation about his health.

KCNA said Kim attended Friday’s ceremony marking the completion of a fertilizer factory near Pyongyang along with senior officials. State TV showed Kim smiling and walking around factory facilities.

Kim earlier vanished from the public eye after presiding over a Politburo meeting of the ruling Workers’ Party on April 11 to discuss the coronavirus. Speculation about his health began swirling after he missed an April 15 event commemorating the birthday of his grandfather and state founder, Kim Il Sung, something he had never done since inheriting power upon his father Kim Jong Il’s death in late 2011.