A serious accident occurred on Wednesday during the launch of a new North Korean warship while leader Kim Jong-un was attending the event, and he called it a "criminal act" that could not be tolerated, state media KCNA reported.

Kim, who witnessed the failed launch of the more-than-4,500-tonne destroyer, said the incident was caused by "carelessness" that tarnished the country's dignity, and ordered the ship to be restored before a key ruling party meeting in June, KCNA said on Thursday.

The report did not mention whether there were any casualties.

KCNA said the incident at the eastern port of Chongjin was caused by a loss of balance while the vessel was being launched and it said sections of the bottom of the warship were crushed, but it did not give more details of damage sustained.

The report noted that Kim "made stern assessment saying that it was a serious accident and criminal act caused by absolute carelessness, irresponsibility and unscientific empiricism which is out of the bounds of possibility and could not be tolerated."

Kim said the accident "brought the dignity and self-respect of our state to a collapse." He said that an immediate restoration of the destroyer was "not merely a practical issue but a political issue directly related to the authority of the state."

South Korea's military said the stricken warship was sideways in the water after the failed launch.

Intelligence authorities in South Korea and the United States had been monitoring the activities in advance, Seoul's Joint Chiefs of Staff spokesperson Lee Sung-jun told a briefing.

The rare public disclosure of an accident followed a report about the launch of another destroyer of a similar size in April at the west coast shipyard of Nampho. Kim also attended that launch. 

North Korea has previously experienced accidents such as space launch vehicle failures and civilian disasters that have subsequently been used to promote the role of the leadership and the ruling Workers' Party in correcting the problems.

The more-than-4,500-tonne destroyers launched by North Korea this year are the country's largest warships yet.

In a report last week on preparations for the most recent launch, U.S.-based 38 North said it appeared the ship would be side-launched from the quay, a method not previously observed in North Korea.

"The use of this launch method could be one of necessity, as the quay where the ship is being built does not have an incline," the 38 North report said.

Commercial satellite imagery of the shipyard the day before the launch showed the destroyer positioned on the quay with support vessels by its side.

A North Korea expert based in Seoul said Pyongyang's disclosure was surprisingly swift.

"It shows again Kim Jong-un's ruling style of cutting off negative rumours from spreading and controlling officials more forcefully by being open about it rather than hiding it," said Cheong Seong-chang of the Sejong Institute.