The United States is deploying a military contingent to a base in Saudi Arabia that has been abandoned for the past 15 years, officials say, marking a major escalation in tension with Saudi's regional arch-nemesis Iran.

The forces deploying to Prince Sultan Air Base, 85 miles south of Riyadh, include fighter jets, air defense missiles and likely more than 500 troops, U.S. officials said on Friday.

Senior American defense officials said some U.S. troops and Patriot air defense missile systems have already arrived at the base, which was abandoned by Washington after it toppled Iraqi strongman Saddam Hussein in 2003.

'In coordination with and at the invitation of the Kingdom of Saudi Arabia, the Secretary of Defense has authorized the movement of U.S. personnel and resources to deploy to Saudi Arabia,' U.S. Central Command said in a statement.

'This movement of forces provides an additional deterrent, and ensures our ability to defend our forces and interests in the region from emergent, credible threats,' Central Command said.

A U.S. airman is seen towing equipment from a transport aircraft on Prince Sultan Air Base in 1996. The U.S. is sending American forces back to the base after abandoning it 15 years ago

The Stena Impero oil tanker has been seized by Iranian authorities while passing through the Strait of Hormuz on Friday, marking a major escalation of tensions

Prince Sultan Air Base at one time housed 60,000 U.S. troops, but has been empty since 2003

History of U.S. troop deployments to Saudi Arabia Saudi Arabia is the nexus of Sunni Muslim power in the Middle East, and is unofficially allied with Israel against their mutual rival, the Shiite Muslim powerhouse Iran. Beginning during Operation Desert Shield in August 1990, while preparing for the Gulf War, the U.S. sent a large troop contingent to Saudi Arabia. At one time, Prince Sultan Air Base housed as many as 60,000 U.S. troops, supported by an all-American array of creature comforts like fast-food restaurants and swimming pools. Following the Gulf War, thousands of U.S. troops remained in Saudi Arabia, many of them supporting the enforcement of no-fly zones over Iraq. The presence of U.S. troops in Saudi Arabia, which is home to the holiest sites in Islam, provoked vicious objections from Sunni fundamentalists, and was the main stated motivation behind the 9/11 attacks. U.S. troops remained in Saudi to support the invasion of Iraq in 2003, but following the end of major operations at the outset of the Iraq War the forces in Saudi were quickly withdrawn. Since 2003, Prince Sultan Air Base has been abandoned, and the only U.S. troops in Saudi Arabia have been a handful of technical and training personnel housed on Saudi-owned bases. Advertisement

Officials said that the troop movement had been in the works for weeks, and was not a response to specific Iranian provocations - though the announcement came just hours after Iran seized two British oil tankers in the Persian Gulf.

President Donald Trump said that the seizures were a mistake and that Iran was playing a dangerous game.

'Iran is showing their true colors,' he told reporters on the South Lawn of the White House has he departed for a weekend in New Jersey. 'Iran is in big trouble.'

The president warned that although the U.S. doesn't have many tankers in the Gulf because it is trending toward energy independence, 'we have a lot of ships there that are warships.'

He predicted the situation 'is going to work out very nicely,' in part because of an informal but 'longstanding' U.S.-UK mutual defense agreement.

'We have no written agreement, but we have an agreement,' Trump said of Britain. 'They've been a very great ally of ours. ... We'll be working with the U.K. They'll have a new prime minister soon, which is a good thing.'

The president warned that although the U.S. doesn't have many tankers in the Gulf because it is trending toward energy independence, 'we have a lot of ships there that are warships'

The president gloated that Iran's belligerence is a sign he was right to distrust Tehran's mullahs in the way his predecessor did, regarding Iran's nuclear ambitions.

'This only goes to show what I'm saying about Iran. Trouble, nothing but trouble,' he said. 'And remember this, the agreement, the ridiculous agreement made by President Obama, expires in a very short period of time.

'It was a short term agreement. When you're dealing in countries, you have to deal in 50 years and 100 years.'

The president's statement came as tensions in the region increase dramatically.

The first vessel, the Stena Impero, was surrounded by heavily-armed Iranian vessels and a helicopter around 4pm and ordered to turn north.

The second vessel, the Mesdar, was ordered by Iran to turn north around 45 minutes later.

Theresa May is chairing an emergency meeting of the COBRA committee at Number 10, which will be attended by Foreign Secretary Jeremy Hunt, who is currently battling with his predecessor Boris Johnson to replace Mrs May as Prime Minister.

The Stena Impero was surrounded by Iranian Revolutionary Guard forces and ordered to head north. The British registered vessel switched off its maritime tracker a short time later. A second British vessel, Medsar, abruptly changed course towards Iran about 45 minutes later

A second UK-owned vessel, the Mesdar, turned dramatically towards Iran around 45 minutes after the Stena Impero was seized

The Iranian Revolutionary Guard uses a large number of high-speed small vessels to harass shipping in the Strait of Hormuz. The regime launched these 'ultra-fast' boats in 2010

Mr Hunt said: 'I will shortly attend a COBR meeting to review what we know and what we can do to swiftly secure the release of the two vessels - a British-flagged vessel and a Liberian-flagged vessel.

'Their crews comprise a range of nationalities, but we understand there are no British citizens on board either ship.

'Our ambassador in Tehran is in contact with the Iranian Ministry of Foreign Affairs to resolve the situation and we are working closely with international partners.

'These seizures are unacceptable. It is essential that freedom of navigation is maintained and that all ships can move safely and freely in the region.'

Iranian authorities have yet to comment on the Mesdar, but marine tracking data showed it turning dramatically off course and towards the Iranian coast.

President Donald Trump said America would be 'working with the UK'.

He told reporters: 'We will talk to the UK and we have no written agreement but we have an agreement. They've been a very great ally of ours.

'So we heard about it, we heard it was one, we heard it was two, and we will be working with the UK.'

Labour deputy leader Tom Watson urged caution in a series of tweets: 'These reports are a matter of real concern. We know that some days ago there was an attempt to steer a British tanker towards Iranian waters.

'Our ships have the right of safe passage and the Stena Impero must be allowed to continue its voyage unimpeded.

'Any move to seize a British tanker would be a significant and harmful escalation of a situation where de-escalation is needed.'

Donald Trump told reporters at the White House that he will soon talk to UK authorities about Iran

The vessel had left Fujairah in Dubai and moved into the Strait of Hormuz when it was intercepted.

According to Iranian state TV, the Stena Impero tanker 'was confiscated by the Revolutionary Guards at the request of Hormozgan Ports and Maritime Organisation when passing through the strait of Hormuz, for failing to respect international maritime rules.'

It is understood the British-flagged tanker was surrounded by small craft and helicopters and ordered to turn north into Iranian waters.

A statement from Stena Bulk, the company which owns the tanker, said the firm had lost contact with the crew of 23 after it was approached by 'unidentified small crafts and a helicopter' at around 4pm.

This image, released from Iranian state TV shows Revolutionary Guard speedboats harrying a Panamanian oil tanker on July 14

Royal Marine Commandos intercepted the Grace I off the coast of Gibraltar, on July 14. The vessel, which is Iranian owned is suspected of smuggling oil to Syria in breach of EU sanctions

According to Marine Traffic, the Mesdar turned dramatically north, pictured

The statement said: 'Stena Bulk and Northern Marine Management can confirm that at approximately 1600 BST on 19th July UK registered vessel Stena Impero (built 2018, 49,683 DWT) was approached by unidentified small crafts and a helicopter during transit of the Strait of Hormuz while the vessel was in international waters.

'We are presently unable to contact the vessel which is now heading north towards Iran.

'There are 23 seafarers aboard. There have been no reported injuries and their safety is of primary concern to both owners and managers.

'The priority of both vessel owner Stena Bulk and ship manager Northern Marine Management is the safety and welfare of the crew.'

Last week, the Royal Navy warship frigate HMS Montrose drove off three Iranian vessels which tried to stop the commercial ship British Heritage as it sailed through the Strait of Hormuz.

Fears were raised that the Iranian authorities were trying to seize a UK ship in retaliation for the detention of the Grace 1 tanker.

The Iranian ship was detained off the coast of Gibraltar on July 4 after it was suspected of violating EU sanctions by carrying a cargo of crude oil destined for Syria.

The ship's captain, chief officer and two second officers were arrested and bailed and an investigation is ongoing.

Iran's supreme leader Ayatollah Ali Khamenei called the tanker's seizure an act of 'piracy' on Tuesday and warned the UK to expect a response.

Foreign Secretary Jeremy Hunt offered to help release Grace 1 if Iran guaranteed it would not breach sanctions imposed on Bashar Assad's regime.

Iran denied claims by President Trump that the USS Boxer, pictured in footage captured by an Iranian drone, shot down the un-manned aircraft over the Strait of Hormuz

The Boxer was armed with a Light Marine Air Defense Integrated System (LMADIS) counter-unmanned aircraft system mounted on a vehicle parked at the bow

A Ministry of Defence spokesman said: 'We are urgently seeking further information and assessing the situation following reports of an incident in the Gulf.'

The UK Chamber of Shipping demanded extra protection for merchant vessels operating in the Persian Gulf.

Bob Sanguinetti, CEO of the Chamber said: 'We condemn unreservedly the capture of Stena Impero as she transited the Strait of Hormuz earlier today.

'This incident represents an escalation. Whilst we call for measured response, it is also clear that further protection for merchant vessels must be forthcoming to ensure enhanced security to guarantee free flow of trade in the region.'

The incident came as Iran and the United States emphatically disagreed Friday over Washington's claim that a U.S. warship downed an Iranian drone near the Persian Gulf. American officials said they used electronic jamming to bring down the unmanned aircraft, while Iran said it simply didn't happen.

Neither side provided evidence to prove its claim.

At the White House on Friday, President Donald Trump said flatly of the Iranian drone: 'We shot it down.' But Pentagon and other officials have said repeatedly that the USS Boxer, a Navy ship in the Strait of Hormuz, actually jammed the drone's signal, causing it to crash, and did not fire a missile. The officials spoke on condition of anonymity to discuss sensitive technology.

Trump's national security adviser, John Bolton, said, 'There is no question this was an Iranian drone, and the USS Boxer took it out as the president announced yesterday because it posed a threat to the ship and its crew. It's entirely the right thing to do.'

In Tehran, the Iranian military said all its drones had returned safely to their bases and denied there was any confrontation with the Boxer, an amphibious assault ship.

The Royal Navy currently has HMS Montrose on patrol in the Persian Gulf

It will soon be joined in the Gulf by the destroyer HMS Duncan

Earlier today, Gibraltar's Supreme Court announced it would extend by 30 days the detention of an Iranian tanker seized two weeks ago on allegations that it was headed to Syria in violation of sanctions.

British authorities' detention of the Grace 1 supertanker sparked outrage in Tehran, which accused London of doing the bidding of the Washington in action that is 'tantamount to maritime banditry'.

On Tuesday, Iran's supreme leader, Ayatollah Ali Khamenei, accused the 'vicious British' of 'piracy' and vowed retaliation.

The Guards also seized another 'foreign tanker' on Thursday, believed to be the Panamanian-flagged vessel Riah and its crew, and accused the ship of smuggling Iranian fuel.

A series of such incidents have sent tensions soaring between Iran on one side and the U.S. and its allies on the other, raising fears of a regional war in the Gulf.

The Strait of Hormuz is the conduit for nearly a third of the world's crude oil.