Iran has seized an oil tanker destined for Turkey, with 19 crew onboard, 18 of whom are Filipinos.

Iran said the seizure of the Marshall Islands-flagged tanker St. Nikolas was in response to the US confiscating the same vessel and its oil last year in a sanction enforcement operation when it sailed under a different name.

Washington has condemned the seizure.

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The Philippine Department of Foreign Affairs has reached out to Iran to check on the condition of the Pinoy seafarers onboard the oil tanker.

"The Department has relayed to the Iranian authorities our concern over the welfare and safety of our 18 crewmen.," said Foreign Affairs Usec. Eduardo de Vera. "We are awaiting an official report from our Embassy in Iran and the Iranian Embassy in Manila."

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"The Philippine Government will spare no efforts in effecting the release of our nationals," the DFA said.

"Iran's actions are contrary to international law and threaten maritime security and stability," said Vice Adm. Brad Cooper, the commander of US Naval Forces Central Command and Commander US 5th Fleet.

Iran has responded with tit-for-tat measures in the past after seizures of Iranian oil shipments.

Crippling US sanctions, reimposed following Washington's 2018 withdrawal from a landmark nuclear deal, target Iranian oil and petrochemical sales in a bid to reduce Iran's energy exports.

"The Iranian government must immediately release the ship and its crew," US State Department spokesman Vedant Patel told reporters.

"This unlawful seizure of a commercial vessel is just the latest behavior by Iran or enabled by Iran aimed at disrupting international commerce."

- Communications lost -

Ambrey, a British maritime risk company, said the group which boarded the St Nikolas covered the ship's cameras. A security officer "reported hearing unknown voices over the phone along with the master's voice", it added.

Communications have been lost with the vessel, which was carrying 19 crew -- 18 Filipinos and one Greek -- the tanker's Greece-based management company Empire Navigation told AFP.

The vessel had been loaded with 145,000 tonnes of crude oil in Basra, Iraq and was destined for Aliaga in Turkey via the Suez Canal, Empire added.

Ambrey said the recently renamed tanker was previously prosecuted and fined for carrying sanctioned Iranian oil, which was confiscated by US authorities.

IRNA, quoting the Iranian navy's public relations office, said the ship was "being transferred to the ports of the Islamic republic for delivery to the judicial authorities".

In September, the United States said it had seized the Suez Rajan and its cargo of 980,000 barrels of crude oil months earlier.

The US Department of Justice said at the time that the oil on the Greek-managed tanker was allegedly being sold by Iran's Islamic Revolutionary Guard Corps to China.

Shortly after that seizure, Iran seized two tankers -- the Marshall Islands-flagged Advantage Sweet as it sailed toward the United States in the Gulf of Oman, and then the Greek-owned Niovi, as it travelled from Dubai to Fujairah.

The Gulf of Oman, a key route for the oil industry that separates Oman and Iran, has witnessed a series of hijackings and attacks over the years, often involving Iran.

Shipping in the resource-rich region is also on heightened alert following weeks of drone and missile attacks in the Red Sea by Yemen's Iran-backed Huthi rebels. - With reports from ANC, Agence France-Presse

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