The military failed to identify the small wooden boat -- carrying four North Koreans -- until a civilian alerted the police after it arrived at a port roughly 160 miles east of Seoul on June 15.

Two of the passengers defected to the South, while the other two chose to return to North Korea.

General Lee Jin-sung was fired after an investigation found the military had failed to pick up the vessel, Defence Minister Jeong Kyeong-doo said at a press conference.

"As a failure in security operations is a serious mistake that cannot be accepted in any circumstances, we have decided to strictly censure those involved," he said.

"I'd like to express an apology as the defence chief," he added. "Under no circumstances is failed military surveillance acceptable."

The sacking was confirmed by the South's Defence Ministry in a separate statement saying the government "will dismiss the army's eighth corps commander" over the mistake.

South Korea's major opposition Liberty Korea Party has demanded a parliamentary probe into the case, claiming the army had tried to cover up the botched incident -- a charge Jeong denied.

Later Wednesday, the presidential Blue House said Kim You-geun, a senior official at the National Security Office, had received a "strict warning" from President Moon Jae-in over the incident.

During years of prolonged economic hardship compounded by international isolation, over 30,000 North Koreans have escaped from the reclusive state to the South, according to government data.

Pyongyang is under heavy economic sanctions because of its multiple nuclear tests and long-range missile launches.

It denounces defectors -- who are an important source for accounts of the regime's brutal treatment of its citizens -- as "human scum".

Contact between the North and South has been minimal since February, when a second summit between the US and Pyongyang collapsed without an agreement over possible denuclearisation and sanctions relief.

But on Sunday US President Donald Trump held an impromptu meeting with North Korean leader Kim Jong Un -- becoming the first sitting American leader to step into the North -- raising expectations for the resumption of working-level talks between Washington and Pyongyang in coming weeks.