Iran's official news agency IRNA said the British-flagged oil tanker was seized allegedly due to a collision with an Iranian fishing boat and for "violating" international maritime rules.

Sources in New Delhi confirmed that 18 Indians were among the people on board the oil tanker, Stena Impero.

"We are ascertaining further details of the incident. Our mission is in touch with Government of Iran to secure the early release and repatriation of Indian nationals," External Affairs Ministry Spokesperson Raveesh Kumar said in response to a question on the detention of Indian crew members along with the oil tanker by Iranian authorities.

The Stena Impero was seized on Friday by the Iranian Revolutionary Guard.

"There are 18 Indian and five crew members from Russia, Philipines, Latvia and other countries on board of Stena Impero. The captain is Indian, but the tanker is UK-flagged," IRNA quoted Allahmorad Afifipour, director general of Ports and Maritime Affairs of Hormuzgan province, as saying.

Stena Bulk, a Swedish company which owns the vessel, said they have been unable to contact the ship, which was seized in the Strait of Hormuz, a key waterway in the region.

"Soon after the vessel was approached by unidentified small naval craft and a helicopter during her transit of the Strait of Hormuz in international waters...the vessel suddenly deviated from her passage to Jubail and headed north towards Iran," the company said in statement.

The vessel was in full compliance with all navigation and international regulations, it said.

Erik Hanell, President and Chief Executive, Stena Bulk, said, "There are 23 seafarers onboard of Indian, Russian, Latvian and Filipino nationality.  There have been no reported injuries and the safety and welfare of our crew remains our primary focus."

"We are in close contact with both the UK and Swedish government authorities to resolve this situation and we are liaising closely with our seafarers' families," Hanell said.

In early July, British marines and Gibraltar police seized an Iranian tanker off the Southern coast of the Iberian Peninsula.

Gibraltar claimed that the ship was transporting crude oil to Syria "in violation" of the EU sanctions placed on Damascus, escalating tensions in the Gulf.