Iran warned Monday its seizure of a South Korean tanker in the Gulf must not be politicised, after the US and France urged the Islamic republic to release the ship.

"We have repeatedly told... the intervening parties, whether they are the United States or France, that the case does not concern them at all and that they will not help to solve a technical problem if they politicise it," said foreign ministry spokesman Saeed Khatibzadeh.

The Islamic Revolutionary Guard Corps seized the Hankuk Chemi and arrested its multinational crew of 20 near the strategic Strait of Hormuz one week ago.

The move came as Tehran urged Seoul to release billions of dollars of Iranian assets frozen in South Korea as part of US sanctions.

The United States and France have called for Iran to release the ship.

A US State Department spokesperson called the seizure "part of a clear attempt to extort the international community into relieving the pressure of sanctions".

The French foreign ministry said the seizure was "fuelling tensions in the region". 

South Korean Deputy Foreign Minister Choi Jong-kun arrived in Tehran on Sunday for a long-planned visit.

The South Korean news agency Yonhap has said Choi's aim during his visit was to "negotiate an early release" of the tanker and its crew, which includes South Korean, Indonesian, Vietnamese and Burmese sailors.

But according to Iran's foreign ministry the "main goal" of his visit was "to discuss ways of accessing Iranian funds in (South) Korea".

Khatibzadeh said on Monday that the South Koreans "had questions about technical problems related to the ship which we answered", without elaborating.