The US is to send 1,000 more troops to the Middle East for what were described as defensive purposes, over concerns about a threat from Iran.

Fears of a confrontation between Iran and the United States have mounted since last Thursday when two oil tankers were attacked, which Washington has blamed on Tehran.

Acting US Defense Secretary Patrick Shanahan said: "The recent Iranian attacks validate the reliable, credible intelligence we have received on hostile behaviour by Iranian forces and their proxy groups that threaten United States personnel and interests across the region."

The new US deployment to the Middle East is in addition to a 1,500-troop increase announced last month in response to tanker attacks in May.

Mr Trump's administration has accused Iran of being behind the explosions on tankers in the Gulf of Oman, a vital oil shipping route.

The United States released a video last week it said showed Iran's Revolutionary Guard was behind the latest attacks near the Strait of Hormuz on the Norwegian-owned Front Altair, which was set ablaze, and the Japanese-owned Kokuka Courageous.

Handout photo of damage to Kokuka Courages from US Dept of Defense

The US military released additional imagery yesterday.

"Iran is responsible for the attack based on video evidence and the resources and proficiency needed to quickly remove the unexploded limpet mine," Central Command said in a statement.

A US defence official, speaking on condition of anonymity, said the new deployment included forces who operate the US military’s airborne intelligence, surveillance and reconnaissance capabilities that help detect threats and scoop up sensitive imagery and intelligence.

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The deployment would also involve personnel who can strengthen protection of US troops already deployed to the region.

Iran's armed forces chief of staff, Major General Mohammad Baqeri, denied that it was behind the attacks and said if the Islamic Republic decided to block the strategic Strait of Hormuz shipping lane, it would do so publicly.

The US previously tightened sanctions, ordering all countries and companies to halt imports of Iranian oil or be banished from the global financial system.

Iran said yesterday it would soon breach limits on how much enriched uranium it can stockpile under the deal, which a White House National Security Council spokesman said amounted to "nuclear blackmail".

The 2015 accord, which Iran and the other signatories have maintained following Mr Trump's decision to withdraw from it, caps Iran's stock of low-enriched uranium at 300kg enriched to 3.67%.

But a spokesman for Iran's Atomic Energy Organisation said: "We have quadrupled the rate of enrichment (of uranium) and even increased it more recently, so that in 10 days it will bypass the 300kg limit.

"Iran's reserves are every day increasing at a more rapid rate," he told state TV, adding that "the move will be reversed once other parties fulfil their commitments".

The move further undermines the nuclear pact also signed by Russia, Britain, Germany, China and the European Union, but Iranian President Hassan Rouhani said the collapse of the deal would not be in the interests of the region or the world.

The nuclear deal seeks to head off any pathway to an Iranian nuclear bomb in return for the removal of most international sanctions.

Britain said if Iran breached agreed limits, London would look at "all options".

Israel urged world powers to step up sanctions against Iran swiftly should it exceed the enriched uranium limit.

European Union foreign policy chief Federica Mogherini said, however, the EU would only react to any breach if the International Atomic Energy Agency formally identified one.