China's government called on Indonesia on Wednesday to treat a group of detained Chinese sailors fairly but gave no indication what their oil tanker was doing when it was found at sea transferring fuel from an Iranian ship.

The Iranian-flagged MT Horse and Panamanian-flagged MT Freya were seized Sunday and are suspected of illegally transferring fuel at sea, shutting off their identification systems and other offenses. Indonesian authorities said 36 Iranian and 25 Chinese crew members were detained.

Iran has sold oil on the black market since then-President Donald Trump imposed sanctions in 2018 and threatened to penalize countries that bought Iranian crude. Iranian oil tankers turn off tracking equipment to conceal their destinations.

In this photo released by Indonesian Maritime Security Agency (BAKAMLA), Panamanian-flagged MT Frea, left, and Iranian-flagged MT Horse tankers are seen anchored together in Pontianak waters off Borneo island, Indonesia, Sunday, Jan. 24, 2021. Indonesian authorities said that they seized the two vessels suspected of carrying out the illegal transfer of oil in their country's waters. (Indonesian Maritime Security Agency via AP)

In this photo released by Indonesian Maritime Security Agency (BAKAMLA), Panamanian-flagged MT Frea, left, and Iranian-flagged MT Horse tankers are seen anchored together in Pontianak waters off Borneo island, Indonesia, Sunday, Jan. 24, 2021. Indonesian authorities said that they seized the two vessels suspected of carrying out the illegal transfer of oil in their country's waters. (Indonesian Maritime Security Agency via AP)

Indonesian authorities confirmed to the Chinese Embassy the crew members are “in good condition,” said foreign ministry spokesperson Zhao Lijian.

The embassy asked Indonesian authorities “to investigate the case impartially according to the law, and to ensure the health, safety and legitimate rights and interests of the crew,” Zhao said.

He gave no details about the Chinese ship's owner or destination.

In this photo released by Indonesian Maritime Security Agency (BAKAMLA), Iranian-flagged MT Horse, left, and Panamanian-flagged MT Frea tankers are seen anchored together in Pontianak waters off Borneo island, Indonesia, Sunday, Jan. 24, 2021. Indonesian authorities said that they seized the two vessels suspected of carrying out the illegal transfer of oil in their country's waters. (Indonesian Maritime Security Agency via AP)

In this photo released by Indonesian Maritime Security Agency (BAKAMLA), Iranian-flagged MT Horse, left, and Panamanian-flagged MT Frea tankers are seen anchored together in Pontianak waters off Borneo island, Indonesia, Sunday, Jan. 24, 2021. Indonesian authorities said that they seized the two vessels suspected of carrying out the illegal transfer of oil in their country's waters. (Indonesian Maritime Security Agency via AP)

On Wednesday, both tankers were anchored off Batam Island in the Riau Islands south of Singapore, the Indonesian government said.

In August, U.S. officials said the Trump administration seized 1.1 million barrels of gasoline from four tankers bound from Iran to Venezuela.

In 2018, ships were captured in satellite photos transferring oil to North Korean ships off the Chinese coast in a possible effort to evade U.N. sanctions on North Korea. The Chinese government said it would investigate but has yet to announced results.