South Korea has pledged to make "multi-pronged" efforts to secure the release of a tanker seized by Iranian forces on Monday.

The 17,400-dwt Hankuk Chemi (built 2000) and its 20 crew was moved into Bandar Abbas for an alleged breach of pollution regulations.

South Korean owner DM Shipping has denied this.

"What's most important is the security and safety of the people," a spokesman for the office of South Korean president Moon Jae-in said on Thursday. "[We] will make multi-pronged efforts to get them freed at an early date."

South Korea's embassy in Iran has confirmed that all 20 seafarers are well and unharmed.

The official did not comment on reports that Iranian president Hassan Rouhani had sent two letters to Moon about "bilateral issues", the Yonhap news agency reported.

Oil cash frozen

Media speculation has linked the detention to a pending visit to Iran by a South Korean diplomat and the freezing of $7bn of Iranian oil money in South Korea two years ago.

Moon is said to have replied to the letters but there have been "no concrete actions", a diplomatic source was quoted as telling the Seoul-based Hankyoreh newspaper.

The government has now sent a delegation to Iran in a bid to resolve the tanker issue.

The group is led by Koh Kyung-sok, director general of the foreign ministry's Africa and Middle Eastern affairs.

On Wednesday, France condemned Iran's move and called for the vessel's immediate release.

"This incident is fueling tensions in the region," the French foreign ministry said in a statement. "France calls for the immediate release of the ship and its crew."

The ministry emphasised the need for freedom of navigation.

DM Shipping said on Wednesday that "armed" Iranian forces boarded the vessel in the Strait of Hormuz.

The crew received a 10 to 20 minute warning of the seizure, the company said in a statement to CNN.

Communications cut

DM Shipping said its communication with the tanker was cut off as soon as the detention took place.

Five South Korean nationals are on board, the country's foreign ministry said. Other seafarers are from Indonesia, Myanmar and Vietnam.

The ship was en route to Fujairah, having been underway eastbound at 12.5 knots prior to the incident, from the Petroleum Chemical Quay in Jubail, Saudi Arabia.

Iran said the vessel was seized for "creating environmental and chemical pollution in the Persian Gulf".