Iran’s foreign ministry says the seizure of an Iranian oil tanker by Indonesia on Saturday is not related to a similar incident in the Persian Gulf when Revolutionary Guards impounded a South Korean vessel on January 4.

Iran sized the vessel saying it violated maritime environmental regulations, at the same time it was demanding Seoul to release $7 billion frozen in its banks because of US sanctions. Iran has denied that it seized the ship as a means of pressure on South Korea.

As the news of Indonesia’s seizure of the Iranian tanker came, speculation began that this could be a retaliation for the seizure of the South Korean tanker and its crew, including two Indonesians.

The Spokesman of Iran’s Foreign Ministry Saeed Khatibzadeh in his weekly briefing Monday said Iran has asked Indonesia to provide details about the seizure of the Iranian-flagged vessel.

Indonesia seized the Iranian-flagged MT Horse and the Panamanian-flagged MT Freya vessels over suspected illegal oil transfer in the country's waters.

Khatibzadeh said that the seizure was over a "a technical issue and it happens in shipping field". "Our Ports Organization and the ship owner company are looking to find the cause of the issue and resolve it," Khatibzadeh told reporters.

Asked to comment on the seized tanker, Iran's Oil Minister Bijan Zanganeh told reporters : "It was carrying oil ... the issue is being followed up by Iran."

Coast guard spokesman Wisnu Pramandita told Reuters that the tankers, seized in waters off Kalimantan province, will be escorted to Batam island in Riau Island Province for further investigation.

"The tankers, first detected at 5:30 a.m. local time (2130 GMT on Jan. 23) concealed their identity by not showing their national flags, turning off automatic identification systems and did not respond to a radio call," Wisnu said in a statement on Sunday.

Wisnu told Reuters on Monday that the ships were "caught red-handed" transferring oil from MT Horse to MT Freya and that there was an oil spill around the receiving tanker.

After former President Donald Trump exited the 2015 Iran nuclear agreement, the United States imposed full sanctions on Tehran's oil exports, cutting off 90 percent of sales. Iran continued limited exports to China often trying to disguise the shipments by turning off tanker identification signals and transferring cargos to other vessels in far locations that took the oil to China as originating in Malaysia or tother countries.

Wisnu added that 61 crew members onboard the vessels were Iranian and Chinese nationals and had been detained. Indonesia's foreign and energy ministries did not immediately comment on the matter.

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.

Both supertankers, each capable of carrying 2 million barrels of oil, were last spotted earlier this month off Singapore, shipping data on Refinitiv Eikon showed.

Very Large Crude Carrier (VLCC) MT Horse, owned by the National Iranian Tanker Company (NITC), was almost fully loaded with oil while VLCC MT Freya, managed by Shanghai Future Ship Management Co, was empty, the data showed.

Iran 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 U.S. sanctions.

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

Over the past few months, MT Freya has delivered two crude oil cargoes totaling about 4 million barrels into Qingdao port on the east coast of China and northeast Yingkou port, said Emma Li, a senior crude analyst with Refinitiv.

The Qingdao cargo was declared as Upper Zakum crude produced in the United Arab Emirates, said Li, who tracks China-destined crude oil shipments. 

With reporting by Reuters