A British-flagged oil tanker was seized by Iran on Friday night, in a major escalation of tensions along one of the world's most vital oil shipping routes. The Stena Impero had been en route to Saudi Arabia but made an abrupt change of course and began moving towards the Iranian island of Qeshm, according to data relayed by maritime tracking services. The ship 'went dark', meaning its identification system was turned off, at 16:29 UK time and nothing has been heard from her or her 23 crew since. Northern Marine, a Clyde-based subsidiary of the ship's Swedish owner Stena AB, confirmed that a 'hostile action' had preceded the vessel's change of course on Fridayafternoon. They issued a statement saying it had been 'approached by unidentified small crafts and a helicopter during transit of the Strait of Hormuz while the vessel was in international waters.' The ship turned suddenly into Iranian waters Credit: marinetraffic.com/PA Iran's Revolutionary Guards said in a statement that they stopped the tanker at the request of the maritime authority in the Iranian province of Hormozgan on suspicion it has "violated international maritime law", but did not elaborate. There were also concerns about a second oil tanker, the British-operated, Liberian-flagged Mesdar, which turned sharply north towards Iran's coast, about 40 minutes after the Stena Impero's course shift. There was no immediate word from the Guards about the second tanker or from the operator of the second tanker on what had prompted the change in direction along the vital international oil shipping route. Tracking data showed the Stena Impero was in the same area where a United Arab Emirates-based vessel was detained on Sunday and where a British vessel, the British Heritage, was blocked by Iranian forces earlier this month. A Cobra meeting was held between officials from the Foreign Office, Ministry of Defence and other Government departments on Fridaynight to determine the UK's response. A Whitehall source told the Telegraph of the Stena Impero: "It does look like it has been hijacked. Ships don't follow that pattern. It turned right and straight into Iranian waters. It is really concerning that this has happened. "It looks on the face of it as though the Iranian Revolutionary Guard have boarded and taken a UK-flagged ship. It appears to be linked to events around the Grace 1 tanker." British authorities seized the Iranian Grace 1 supertanker off the coast of Gibraltar on July 4, on suspicion it was carrying crude to Syria in violation of European Union sanctions. The fate of the tanker has been at the centre of escalating tensions between the UK and Iran and seen as a pawn in the standoff between the Islamic Republic and the West. Jeremy Hunt, Foreign Secretary, had hinted last Saturday that the UK would release the ship if Iran promised its cargo would not go to the Syrian regime. He said talks between him and counterpart Mohammad Javad Zarif had been productive. However, a court in Gibraltar on Friday extended for 30 days the detention of the vessel, which was carrying two million barrels of oil. Revolutionary Guards have been threatening retaliation for its impounding and the move would likely have aggravated an already-tense situation. Tensions have been building for weeks in the Persian Gulf. On 10 July, a British warship, the HMS Montrose, intervened to drive three Iranian military vessels that were attempting to divert the British Heritage. Iran seized a Panama-flagged ship on Sunday, it alleges, for 'smuggling oil to foreign countries". However, mystery has surrounded the capture as no country has come forward to claim the ship or its cargo. The vessel, however, was only carrying a very small amount and it had been thought Iran had seized it as merely a show of strength. The US then on Thursday claimed to have downed an Iranian drone that had been flying too close to one of its navy ships. The USS Boxer, an amphibious assault craft, destroyed the drone after it came within 1,000 yards in the Strait of Hormuz, at the entrance to the Gulf However, Iran denied the claims and released footage on state TV to proof it was still in possession of the drone. The latest incidents will only increase fears for security along the Strait of Hormuz, through which almost one-fifth of the world's oil passes. Oil prices rose on Fridaynight in reaction to the news. After one of the worst performing weeks since May, oil started the day firmer but slipped as the US and Iran continued to trade brickbats. The later rise initially still left it well down on the previous week. Oil was down more than 8pc this week overall when markets in London closed. Iran has threatened to close the Strait if it cannot export its oil. The Trump administration is trying to block Iran's exports as a way to pressure it to renegotiate the landmark 2015 nuclear deal it abandoned last year. The UK, which is understood to have seized the Grace 1 after a request from the US, is trying - alongside the EU - to keep the accord alive, believing it is the best chance to stop Tehran acquiring a nuclear weapon. Click here to read full news..