South Korea will send a delegation to Iran to negotiate the release of a seized oil tanker and its crew, Seoul said Tuesday, as a anti-piracy unit arrived in waters near the Strait of Hormuz.

Iran's Revolutionary Guards said Monday it had seized the South Korean-flagged Hankuk Chemi -- which it said was carrying 7,200 tonnes of "oil chemical products" -- for infringing maritime environmental laws.

Seoul's defence ministry said Tuesday a destroyer carrying members of South Korea's anti-piracy unit had arrived in waters near the Strait of Hormuz and was "carrying out a mission to ensure the safety of our nationals", without giving details.

Seoul said the 300-strong Cheonghae unit had been in the region since late last year and would not engage in an offensive operation, an unnamed military official told the South's Yonhap News Agency.

"The issue should be resolved through diplomacy. The unit is focused on the safety of our people who use the waterway after the seizure incident," they added.

Foreign ministry spokesman Choi Young-sam said a government delegation would be "dispatched to Iran at the earliest possible date to try to resolve the matter through bilateral negotiations."

The arrested crew were from South Korea, Indonesia, Vietnam, and Myanmar, the Guards said on its website Sepahnews, without giving further details.

The seizure followed days of high US-Iran tensions marked by the first anniversary of the US assassination of Iran's revered military commander Major General Qasem Soleimani in a Baghdad drone strike.

The United States Sunday reversed a decision to bring the aircraft carrier USS Nimitz home from the Gulf, with the Pentagon citing "recent threats" by the Islamic Republic.

On Monday, Iran said it had started the process to enrich uranium to 20 percent purity, a move that quickly drew international concern.

- Billions of dollars locked -

Separately, South Korea's vice foreign minister Choi Jong-kun will go ahead with a planned three-day trip to Tehran early next week, the spokesman added.

The vice minister's visit had been arranged prior to the seizure, as Tehran seeks the release of billions of dollars held in Seoul under US sanctions.

According to Iran's central bank governor Abdolnasser Hemmati, the country has "$7 billion of deposits in South Korea" that can neither "be transferred nor do we get any returns on, while they ask us for the costs" of holding the funds.

South Korea's foreign minister Kang Kyung-wha did not comment on speculation that Iran seized the ship in a bid to pressure Seoul to unlock Iranian assets.

"We need to verify the facts first and ensure the safety of our crew," Kang told reporters.

"We are making diplomatic efforts for an early release," she added.

Iran says South Korea holding its $7b funds 'hostage'

Iran on Tuesday accused South Korea of holding $7 billion of its funds "hostage" but stressed that Tehran's seizure of a Korean tanker in Gulf waters was not a tit-for-tat move.

The Islamic Revolutionary Guard Corps on Monday sent out naval speedboats to seize the Hankuk Chemi with 20 crew aboard, accusing it of "breaking maritime environmental law".

South Korea has demanded the release of the ship and dispatched its anti-piracy unit to the region.

"We are not hostage-takers," Iran's government spokesman Ali Rabiei told a press conference. "We are used to such allegations. But ... it is the government of Korea that has taken over $7 billion of ours hostage on baseless grounds".

He added that the seizure was based on a court order after the tanker had "caused oil pollution in the Persian Gulf. It was warned beforehand, and the (seizure) request was a technical one."

South Korea will dispatch a delegation to Iran soon to resolve the tanker situation "through bilateral negotiations," said its foreign ministry spokesman Choi Young-sam.

South Korea's vice foreign minister Choi Jong-kun will go ahead with a previously planned three-day trip to Tehran early next week, the spokesman added.

The vice minister's visit had been arranged prior to the seizure, as Tehran seeks the release of the billions of dollars held in Seoul over US sanctions.

Rabiei said South Korea had refused to provide Iran with the funds "even regarding goods that are not sanctioned" by the United States, noting that "our nation's historical memory will not forget this".

US President Donald Trump in 2018 withdrew Washington from a landmark nuclear agreement with world powers and then reimposed and reinforced crippling sanctions on Iran.

Iran's Guards said the Korean vessel was carrying 7,200 tonnes of "oil chemical products" after stopping at a Saudi port and was crewed by personnel from South Korea, Indonesia, Vietnam and Myanmar.

The incident was the first seizure of a major vessel by the Iranian navy in more than a year.

In July 2019, the Guards seized the British-flagged oil tanker Stena Impero in the sensitive Strait of Hormuz for two months for allegedly ramming a fishing boat.

It was at the time widely seen as a response after authorities in the British overseas territory of Gibraltar had detained an Iranian tanker and later released it over US objections.

Tehran denied the two cases were related.