US President Donald Trump has claimed that Iran had "taken some shots" at the Panama-flagged vessel and urged South Korea to join American operations aimed at restoring normal shipping through the strait.

The vital waterway has been virtually closed since the United States and Israel launched a war against Iran on February 28, with Tehran responding with attacks across the Middle East and imposing a chokehold on the strait that has rattled global energy markets.

"... two unidentified aircraft struck the outer plate of the port-side ballast tank at the stern of the HMM Namu at roughly one-minute intervals, causing flames and smoke," foreign ministry spokesman Park Il told a news briefing about Monday's attack.

He did not specify what kind of aircraft were involved, saying they "were captured on CCTV footage, but there are limitations in identifying the exact type, launch origin and physical size of the objects".

Tehran has denied responsibility for the attack, with its embassy in Seoul posting a statement on its website saying that it "firmly rejects and categorically denies any allegations regarding the involvement" of its forces.

Iran's ambassador to South Korea was summoned to the foreign ministry to explain Tehran's position regarding the findings, Park said.

He declined to speculate about how the government might respond if Iran was found to be responsible for the attack.

- Twisted metal -

The damage to the ship, which had 24 crew members on board, spanned about "five metres (16.5 feet) wide and extended roughly seven metres (23 feet) into the hull" on the port-side stern, Park said.

"The engine room fire is believed to have been ignited by the first strike, with the second strike appearing to have caused the blaze to spread rapidly," he said.

Park said further analysis will be conducted on "engine debris and other fragments".

A photo released by the foreign ministry shows the ship's engine room heavily burned.

Another photo shows a large breach in the outer hull near the stern, with twisted metal and internal frames visible around the impact area.

South Korea was reviewing participation in US-led initiatives related to freedom of navigation, including the US-led Maritime Freedom Construct, or "Project Freedom", Park said.

Asia's fourth-largest economy relies heavily on Middle Eastern fuel imports, most of which transited through the Strait of Hormuz before it was effectively closed.

South Korea, a major petrochemicals producer and refiner, has since imposed a fuel price cap for the first time in nearly 30 years.

It has also sought to diversify its fuel supply, securing more than 270 million barrels of crude -- sufficient for more than three months of its oil needs -- via routes unaffected by the blockade.