Royal Marine patrol vessel is seen beside the Grace 1 super tanker in the British territory of Gibraltar,  July 4, 2019.
FILE - Royal Marine patrol vessel is seen beside the Grace 1 super tanker in the British territory of Gibraltar, July 4, 2019.

The impounding off the coast of Gibraltar of a supertanker suspected of smuggling two million barrels of Iranian oil is adding to spiraling tensions between Washington and Tehran. 

Iranian officials say the seizure this week by British royal Marines, of a ship that appeared to be heading for Syria, was at the behest of the U.S. The impounding of the supertanker was quickly welcomed by White House national security adviser John Bolton.

"Excellent news: UK has detained the supertanker Grace I laden with Iranian oil bound for Syria in violation of EU sanctions," Bolton tweeted.

Western diplomats are braced for Iranian reprisals, most likely in the Strait of Hormuz. 

Iran has already been accused by Washington of sabotaging half-a-dozen oil tankers in the strait in a policy of brinkmanship aimed at underlining to European countries the costs of siding with Washington in a confrontation triggered by U.S. President Donald Trump's decision last year to abandon the 2015 nuclear deal and reimpose economic sanctions. 

Iran has denied it has mounted any mining operations against shipping in the Strait of Hormuz.

A detachment of 30 British Royal Marines working with Gibraltar's police intercepted Grace 1 overnight Wednesday as it headed east off the British peninsula. Some marines landed on the deck by dropping from ropes suspended below a hovering Wildcat helicopter. Others boarded from speedboats, pulled up alongside. 

FILE - The oil supertanker Grace 1, that's on suspicion of carrying Iranian crude oil to Syria, sits anchored in waters of the British overseas territory of Gibraltar, historically claimed by Spain, July 4, 2019.

Britain's Ministry of Defense released black-and-white thermal images of the helicopter over the ship's bow, saying the operation was carried out at the request of Gibraltar authorities.

Arguments from Spain, Iran, Britain

But Spain's foreign minister, Josep Borrell, said he believed the dramatic maneuver had been conducted at the request of the U.S. Spain has criticized the impounding, claiming the vessel was technically in Spanish waters. 

Tehran responded Thursday by summoning Britain's ambassador to its foreign ministry to explain what it described as an "illegal seizure." But British officials say they had a legal duty to seize the vessel, which is Iranian-owned and had been Panamanian-flagged until the end of May. They say they were enforcing European Union sanctions against the Syrian government of Bashir al-Assad. 

FILE- In this picture released by an official website of the office of the Iranian supreme leader, Supreme Leader Ayatollah Ali Khamenei, right, arrives at a graduation ceremony of the Revolutionary Guard's officers, in Tehran, Iran.

The vessel, according to British officials, was acting flagrantly. The ship's captain recorded Grace 1 as having taken on the oil from an Iraqi port, but tracking websites indicate the ship did not dock at an Iraqi port. The vessel also took a long route, avoiding the heavily-monitored Suez Canal, instead sailing around the Cape of Good Hope. To the surprise of British authorities, the ship's captain kept his vessel's communications transponders on as it neared Gibraltar. 

Fabian Picardo, Gibraltar's chief minister, said, "This action arose from information giving the Gibraltar government reasonable grounds to believe that the vessel, the Grace 1, was acting in breach of EU sanctions against Syria. In fact, we have reason to believe that the Grace 1 was carrying its shipment of crude oil to the Banyas refinery in Syria."

Calls for 'de-escalation'

Tehran is suspected of smuggling crude oil exports to China, but most of its oil revenue has dried up since the reimposition of U.S. sanctions.

The British move is being seen by analysts as adding to signs that London is stiffening its attitude toward Tehran in the confrontation between the U.S. and Iran, the result, diplomats say, partly of talks during the American leader's recent visit to London, but also because of aggressive Iranian actions.

Britain has been calling for a "de-escalation" in the Persian Gulf and has been working alongside France and Germany to find ways to bypass the reimposed U.S. sanctions, but it has been more forthright than France or Germany in condemning Iran for aggression in the Strait of Hormuz, including mining tankers and downing a U.S. drone. 

FILE - Head of the Revolutionary Guard's aerospace division Gen. Amir Ali Hajizadeh looks at debris from what the division describes as the U.S. drone which was shot down in Tehran, Iran, June 21, 2019.

The British have also been more outspoken in condemning Tehran for its threats to step up nuclear activities and its breaching of the cap on uranium stockpile limits set by the 2015 accord.

The 2015 deal was co-signed by Trump's predecessor, Barack Obama. Tehran agreed to curb its nuclear program in return for sanctions relief. But President Trump withdrew the U.S. from the agreement, citing concerns that Tehran had done nothing to stop expansionist behavior in the region and was still determined to eventually build nuclear weapons.

Strait of Hormuz

In the wake of the British seizure of Grace 1, attention is turning once again to the Strait of Hormuz, the narrow, strategic sea passage between the Persian Gulf and the Gulf of Oman and one of the world's most important oil arteries. The 39-kilometer-wide and 154-kilometer-long strait is the only sea route to the open ocean for more than one-sixth of global oil production and a third of the world's liquified natural gas.

Analysts at the Eurasia Group, a political risk consulting firm, see rising risks of a military clash. They estimate there's a 40 percent "probability of war between the U.S. and Iran." 

Slow-moving tankers have limited room for maneuver in the congested waterway and are confined to narrow shipping lanes under a traffic separation plan to reduce the chances of collision. That heightens their vulnerability to attack.

During the Iran-Iraq War in the 1980s, Iraqi and Iranian forces engaged in a tanker war in the strait. Iraqi leader Saddam Hussein hoped that by attacking Iranian shipping, it would provoke the Iranians to try to close the Strait of Hormuz to all maritime traffic, prompting, he hoped, a full-scale American intervention. But Iran did not take the bait and limited its reprisals to Iraqi shipping. Even so, the tanker war, which saw 450 vessels struck, raised the cost of insuring vessels and triggered an oil-price rise.

FILE - MH-60S helicopter hovers in the air with an oil tanker in the background as the USS John C. Stennis makes its way to the Gulf through the Strait of Hormuz, Dec. 21, 2018.

In 1988, though, U.S. forces did launch attacks briefly in Iranian territorial waters after an American warship struck an Iranian mine. In a one-day battle, assigned the name Operation Praying Mantis by the U.S., one Iranian frigate, a gunboat, and a half-dozen armed speedboats were sunk by U.S. forces.

During the subsequent months of heightened tensions, the U.S. warship USS Vincennes wrongly identified an Iran Air Airbus A300 as a warplane and shot it down, killing 290 people.

In 2008 and 2012, there were further tensions in the region with Iran threatening to close the strait, but rhetoric was not turned into action. 

General Martin Dempsey, the then-chairman of the Joint Chiefs of Staff, said in 2012 that while Iran "has invested in capabilities that could, in fact, for a period of time block the Strait of Hormuz" the U.S. had "invested in capabilities to ensure that if that happens, we can defeat that."

But since 2012, Iran's Revolutionary Guards have been working on tactics and hardware for a confrontation in the strait. 

One tactic would likely rest with using high-speed and agile small boats to swarm U.S. warships and tankers. Iran has also been developing an arsenal of ballistic missiles, which can be land-launched or fired from submarines. Iran's warplanes are aged U.S. F-14 jets and old Russian MiG-29s and no match for America's advanced fighters, say military analysts, but it does have 32 batteries of the Russian-built S-300 air-defense system.

U.S. officials are confident that they can keep the strait open, although some Pentagon officials and military analysts say shipping could be interrupted for a few days while the U.S. Fifth Fleet gained control of the waterway.

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