North Korea has begun an investigation into an accident that occurred during the launch of a new warship this week, state media said.

North Korea said Thursday that "a serious accident occurred" at the Wednesday launch ceremony for the newly built 5,000-tonne naval destroyer, in which sections of the bottom of the vessel were crushed, with leader Kim Jong-un calling the mishap a "criminal act".

South Korea's military said US and Seoul intelligence authorities assessed that North Korea's "side-launch attempt" of the ship failed, and the vessel was left listing in the water.

However, the North's official Korean Central News Agency (KCNA) said Friday that an "underwater and internal inspection of the warship confirmed that, unlike the initial announcement, there were no holes made at the warship's bottom".

"The hull starboard was scratched, and a certain amount of seawater flowed into the stern section through the rescue channel," according to KCNA.

Criminal act

North Korea on Thursday had blamed "inexperienced command and operational carelessness" for the destroyer's botched launch, which was observed by Kim, who called it a "criminal act caused by absolute carelessness".

Hong Kil Ho, the manager of the shipyard in the eastern port city of Chongjin where the accident took place, was summoned by law enforcement on Thursday, KCNA said on Friday.

Experts estimated it would take "two or three days to keep the balance of the warship by pumping up the seawater from the flooded chamber", the report said.

It would take around 10 days to restore the destroyer's side, it added.

Based on its size and scale, the South Korean military said the newly built warship is believed to be similarly equipped to the 5,000-tonne destroyer-class vessel, Choe Hyon, which North Korea unveiled last month.

North Korea has claimed that Choe Hyon was equipped with the "most powerful weapons" and that it would "enter into operation early next year".