South Korea has asked Qatar for “maximum support” to secure the release of an oil tanker seized by Iran this month.

Deputy Foreign Minister Choi Jong-Kun made the appeal during a visit to Doha, the Qatari capital. He met Foreign Minister Mohammed Bin Abdulrahman Al Thani, the Yonhap News Agency reported on Thursday.

Choi traveled to Doha from Tehran after fruitless talks over the South Korean-flagged Hankuk Chemi, an oil-products tanker detained by the Islamic Revolutionary Guard Corps on Jan. 4. He also discussed billions of dollars of Iranian money trapped in South Korea due to U.S. sanctions.

Qatar shares the world’s biggest natural gas field with Iran and has maintained close ties with the country even as other Gulf Arab nations such as Saudi Arabia and the United Arab Emirates have backed U.S. attempts to isolate it.

Relations between Tehran and Seoul have been strained since the U.S. banned countries, including major Asian customers, from buying Iranian petroleum.

Iran says it has at least $7 billion from oil sales stuck in South Korea and that it needs the money to purchase goods including coronavirus vaccines.

South Korea has shown a “lack of will” to release the oil funds and should avoid “politicizing” the Hankuk Chemi, Iran’s Deputy Foreign Minister Abbas Araghchi said on Sunday.

Iran said the ship was violating marine environmental laws, something its operator has denied. Around 20 crew members are being held with the ship at Iran’s Bandar Abbas port.