SEOUL - South Korea said on Wednesday it sacked an army commander and rebuked other senior officers after a North Korean fishing boat piloted by defectors went undetected in South Korean waters for more than two days last month.

The incident has raised concerns about potential lapses in South Korea's security during fragile talks to end a technical state of war with North Korea since the 1950-53 Korean War.

The small wooden boat spent more than two days in waters south of the Northern Limit Line, which separates areas controlled by the two Koreas, before docking at the port of Samcheok.

The ship's arrival on June 15 was spotted by closed-circuit television cameras, but authorities failed to identify the vessel as North Korean, said Choi Byung-hwan, vice minister of the Office for Government Policy Coordination.

The four fishermen waited, expecting to face South Korean security forces who never came. One man approached a South Korean resident and asked to use a telephone, Choi said, and police were alerted.

The commander of the South Korean Army's 8th Corps, which oversees the area in eastern South Korea where the boat docked, was removed from his command, the defence ministry said.

The chairman of the joint chiefs of staff received a warning, and two other senior military commanders were referred to a disciplinary committee, the ministry said.

"Our analysis found that there was a failure in our security operation," Defence Minister Jeong Kyeong-doo told reporters in the capital Seoul.

"This failure was a grave error that cannot be acceptable in any case," he added.

Two of the fishermen said they wanted to stay in South Korea, officials said, and the other two asked to go home.

At least 546 North Koreans defected to South Korea in the first six months of this year, up from 487 in the same period last year, South Korea's unification ministry said on Tuesday, according to Yonhap news agency.

Most North Korean escapees make their way through China and Southeast Asia, rather than attempting to cross the heavily guarded border between North and South Korea.