Indonesia's coast guard had seized the Iranian-flagged MT Horse and the Panamanian-flagged MT Frea vessels over suspected illegal fuel transfers off the country's waters.

A statement from coast guard spokesman Wisnu Pramandita said on Sunday the tankers, seized in waters off Kalimantan province, were escorted to Batam island in Riau Island Province for further investigation.

"The tankers, first detected at 5:30 AM local time (2130 GMT) concealed their identity by not showing their national flags, turning of automatic identification systems, and did not respond to a radio call," the statement said.

"There was an oil spill around MT Frea," it said.

Iran's oil exports under US sanctions

The International Maritime Organization requires vessels to use transponders for safety and transparency. 

Crews can turn off the devices if there is a danger of piracy or similar hazards. But transponders are often shut down to conceal a ship's location during illicit activities.

Iran, which has not commented on the seizure, has been accused of concealing the destination of its oil sales by disabling tracking systems on its tankers, making it difficult to assess how much crude Tehran exports as it seeks to counter US sanctions.

In 2018, former President Donald Trump pulled Washington out of Iran's 2015 nuclear deal with six major powers and reimposed sanctions aimed at cutting Tehran's oil exports to zero.

Iran sent the MT Horse vessel to Venezuela last year to deliver 2.1 million barrels of Iranian condensate.