SEOUL, South Korea — Iran has agreed to free the 20-member crew of a seized South Korea-flagged ship, both countries said Tuesday, in what appeared to be the first significant gesture by the Iranians to de-escalate the problem since impounding the vessel a month ago.
The Iranian move may also have been intended to send an indirect signal to the Biden administration, which has indicated it wants to avoid a further deterioration in relations with Iran after they worsened sharply under former President Donald J. Trump.
In return for the release of the crew, the South Korean government said it had pledged speedy action to address Iran’s complaints about its inability to access $7 billion in Iranian money that has been frozen in South Korean banks because of American sanctions reimposed by Mr. Trump.
It was not immediately clear from the announcement when the crew members would be released. Iran said the ship and its captain would remain in its custody, pending an investigation into what the Iranians have described as violations of maritime pollution law, the original reason they gave for having seized the vessel.
Laden with 7,200 tons of chemicals, the ship, the Hankuk Chemi, was taken into custody by the Islamic Revolutionary Guards Corps during a patrol in the Persian Gulf on Jan. 4. South Korea strongly protested the seizure, and the ship’s owner called Iran’s accusations absurd.
It soon became clear that Iran had taken custody of the ship at least in part to pressure South Korea, a strong American ally, over the sanctions ordered by Mr. Trump after he renounced the 2015 nuclear accord between Iran and major world powers. Those sanctions included blocking Iran’s access to billions of dollars in Iranian oil revenue deposited in foreign banks.
Iran began disregarding its commitments under the nuclear agreement in response to Mr. Trump’s actions, and has threatened further steps that include possibly blocking international nuclear inspectors from visiting.
While President Biden has said he wants to rejoin the nuclear agreement if Iran resumes compliance, Iran has said the United States must drop the sanctions first. Neither side has publicly shown an immediate willingness to find a diplomatic solution.
But Iran’s agreement to free the South Korean ship’s crew, which Saeed Khatibzadeh, a Foreign Ministry spokesman, described as a humanitarian gesture, appeared intended to portray some measure of flexibility on sanctions-related issues.
“This may be a signal to show a willingness to resume discussions or at the least take the tensions down a notch, and perhaps to open the door for South Korea releasing seized Iranian assets,” said Farhad Alavi, a partner at Akrivis Law Group, a Washington firm that specializes in sanctions law.
“Similarly, I would not be surprised if President Biden lifts or suspends some less sensitive or perhaps more political Trump-era sanctions in the coming weeks or months — something more symbolic than substantial,” Mr. Alavi said.
There was no immediate comment from the Biden administration on the news about the South Korean crew members.
South Korea’s Foreign Ministry said in a statement that the decision to free the crew members came during a telephone conversation on Tuesday between Iran’s deputy foreign minister, Seyed Abbas Araghchi, and his South Korean counterpart, Choi Jong-kun.
The crew includes four South Koreans, and the other members are Burmese, Vietnamese and Indonesian, the statement said.
Mr. Choi welcomed the Iranian decision and urged the Iranian government to release the captain and the ship as well.
During his telephone conversation with Mr. Araghchi, Mr. Choi promised “speedy” action to help address Iran’s grievances over the $7 billion in impounded money.
Mr. Choi also told Mr. Araghchi that South Korea would consult American officials in Washington over the matter, the ministry said.
Choe Sang-hun reported from Seoul, and Farnaz Fassihi from New York. Rick Gladstone contributed reporting from Eastham, Mass.