South Korea has secured "decisive" evidence that North Korea's recent shooting at one of its guard posts across the border was not intentional, sources said Wednesday.

The assessment is in line with the United States' determination that the incident was "accidental."

On May 3, at least four bullets from the North hit the South's guard post at the central part of the Demilitarized Zone in Cheorwon prompting the South Korean troops to fire back, according to the Joint Chiefs of Staff (JCS).

"I cannot tell you details, but we've secured evidence from various intelligence sources, including circumstantial ones, that it was accidental," a senior JCS officer told reporters, explaining the results of its probe into the case.

The official later described the evidence as "decisive."

Other sources said the evidence, which includes signals intelligence (SIGINT), has been shared with the US. On the day of the incident, US Secretary of State Mike Pompeo said in an interview with ABC News that he believes the incident was "accidental."

The JCS officer also pointed out that the North did not take any specific action despite the South's shooting at its guard post.

"If they had recognized the situation seriously, they should have prepared for action, just as we had done at that time. But no such specific moves, like wearing iron helmets, have been detected," he said. "Civilians near the scene were also spotted doing their farm work just as usual in the day."

The shooting took place at a time when North Korean soldiers usually rotate shifts and check their firearms or equipment, raising chances of accidental firings, another JCS officer said.

It was foggy, and the North's guard post is topographically lower than the South Korean one, which are unfavorable conditions for the North to be provocative, he added.

North Korea remains mum, though South Korea sent a notice lodging a strong complaint and calling for an immediate explanation after the incident, according to the defense ministry.

"We communicate with North Korea twice a day via military hotlines, and we raised the issue through this channel before. We are waiting for their responses," a ministry official said.

The US-led United Nations Command, which administers the buffer zone and enforces the Armistice Agreement, said it dispatched a multinational special investigation team to the scene to determine if any violation of the pact that halted the 1950-53 Korean War took place during the gunfire.

"UNC has been looking into the case. After the results come, we can make a final conclusion," the JCS officer noted.

Asked how North Korea hit the South's guard post though they were firing accidentally, another JCS officer said that both sides of the border always point the muzzle of their weapons toward each other.

Officials said the South's response was slow due to a gun malfunctioning.

The South fired back 32 minutes after hearing the North's gunshots and 22 minutes after soldiers on duty found bullet marks on the wall of the post, the JCS officer said.

The soldiers first heard the sound and witnessed a flash at 7:41 a.m.

"Our troops first attempted to fire a KR-6 gun with a remote control system. As it malfunctioned, however, they turned to the K-3 light machine gun," the senior officer said.

The South Korean side fired about 15 rounds with the K-3 machine gun at the North Korean guard post.

Upon learning the North employed a 14.5-millimeter anti-aircraft machine gun, however, the South fired another 15 rounds with a 12.7-mm K-6 heavy machine gun "in accordance with the principle of proportional responses," according to the officer.

"As the North Korean military aimed at our guard post, we also made the same, corresponding action. It was done by our response manual," he noted.

It was the first exchange of gunfire between South and North Korea in years. Under the Comprehensive Military Agreement signed in September 2018, the two Koreas agreed to halt all hostile acts against each other, a move aimed at reducing tensions and building trust.

The DMZ, which is about 250 kilometers long and 4 km wide, is one of the world's most heavily fortified borders and has often been a flashpoint between the rival Koreas, though South Korea has been pushing to turn the zone into a global peace zone. (Yonhap)