Indonesia has seized an Iranian tanker and Panamanian vessel suspected of illegally transferring oil in the Southeast Asian nation's waters, authorities said Monday.

The crude oil tankers -- Iranian-flagged MT Horse and Panama-flagged MT Freya -- were spotted Sunday off Kalimantan, Indonesia's section of Borneo island, and were later seized after the crew failed to respond to radio calls, authorities said.

The crew were suspected of a string of violations, including failing to display the vessels' national flags, turning off their identification systems to avoid detection and illegally transferring oil, the Indonesian Maritime Security Agency said.

"We seized the two foreign tankers yesterday and the investigation is ongoing," said agency spokesman Wisnu Pramandita.

"The crews are being questioned."

The administration of former US president Donald Trump repeatedly accused Iran of trying to conceal its oil sales to avoid crippling American sanctions.

Iran's foreign ministry said it had received "contradictory information" concerning the tankers and was awaiting further clarification.

Tehran has been told the vessels were seized over "a technical issue", ministry spokesman Saeed Khatibzadeh told reporters.

He declined to comment on a question over possible links between Monday's incident and Iran's seizure of a South Korean ship earlier this month.

Iran's Oil Minister Bijan Namdar Zanganeh confirmed that the MT Horse carried Iranian oil, state news agency IRNA reported, and said "the situation is being followed up on".

Iran's Revolutionary Guards seized the South Korean-flagged tanker Hankuk Chemi and arrested its multinational crew of 20 near the strategic Strait of Hormuz on January 4.

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

In October, the Trump administration slapped fresh sanctions on Iran's oil sector over sales to countries including Syria and Venezuela, on top of broad sanctions reimposed from 2018 after Washington quit a nuclear accord with major powers and Iran.

The move was part of a broader bid to stop Iran's key oil exports, seeking to choke off all cash sources for the regional nemesis of US allies Saudi Arabia and Israel.

In July 2019, the Guards seized the British-flagged oil tanker Stena Impero in the strait for allegedly ramming a fishing boat and released it two months later.

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

Tehran denied the two cases were related.

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