SEOUL, South Korea — The South Korean government apologized on Thursday for a security lapse that allowed a North Korean fishing boat to spend two and a half days in its waters without being noticed.

“What has been revealed so far is enough to cause deep concern among the people,” Prime Minister Lee Nak-yon said. “We offer our deep apologies.”

The 33-foot wooden boat crossed the maritime border with South Korea last week with four North Korean men on board. Neither the South Korean Navy nor the Coast Guard knew about it until Sunday morning, when the boat docked at Samcheok, a port 80 miles south of the border.

One of the four North Koreans then came ashore, telling a South Korean villager that he came from the North and asking to borrow a cellphone so he could call an aunt who had earlier defected to the South.