SEOUL (Reuters) - North Korea threatened retaliation on Friday after South Korea fired what it said were warning shots when a patrol boat and fishing boat from the North crossed the disputed sea border off the west coast of the Korean peninsula.

North Korean leader Kim Jong Un attends the first congress of the country's ruling Workers' Party in 36 years, in Pyongyang, North Korea, in this photo taken by Kyodo May 9, 2016. Mandatory credit Kyodo/via REUTERS

The two vessels from the North retreated about eight minutes after the South Korean navy fired five 40 mm artillery shots at around 7:30 a.m. local time, South Korean officials told Reuters.

The North Korean boats had crossed the Northern Limit Line, a border that the North disputes, near the South Korean border island of Yeonpyeong, according to the South Korean military.

North Korea accused the South Korean navy of intruding into its waters and said the South fired at its ships in a “grave provocative act,” the Supreme Command of the North’s Korean People’s Army was quoted as saying by the official KCNA news agency late on Friday.

“The provocation-makers are going to regret for ever how horrible the aftermath of their reckless firing first will be,” it was quoted as saying.

North Korea frequently makes threatening statements against the South. Tensions have been high since the North conducted a nuclear test in January and a space rocket launch in February, prompting a United Nations Security Council resolution in March tightening sanctions against the isolated state.

North Korean fishing boats occasionally stray into South Korean waters. Over the years, navy vessels from both sides have traded fire in sometimes deadly incidents.

In 2010, 46 South Korean sailors were killed when their ship sank in what the South says was a torpedo attack by the North. North Korea has denied responsibility.

The two countries remain in a technical state of war since their 1950-53 conflict ended in a truce, not a peace treaty.

Pyongyang recently proposed military talks with Seoul, but the South dismissed the offer as “a bogus peace offensive” because it lacks a plan to end the North’s nuclear program.