Acting US Defence Secretary Patrick Shanahan has announced the deployment of about 1,000 more troops to the Middle East for what he says are "defensive purposes", citing concerns about a threat from Iran.

Key points: The US military says some photos show Iranian forces removing an unexploded mine from the side of an oil tanker

The US military says some photos show Iranian forces removing an unexploded mine from the side of an oil tanker The US Government is working to convince members of Congress and allies Tehran is responsible for the attacks

The US Government is working to convince members of Congress and allies Tehran is responsible for the attacks Two US officials told Reuters the US was preparing to send additional troops to the Middle East in response to mounting concerns over Iran

Fears of a confrontation between Iran and the United States have mounted since last Thursday when two oil tankers were attacked in the Gulf of Oman.

The US previously tightened its sanctions against Iran, ordering all countries and companies to halt imports of Iranian oil or be banished from the global financial system, a move aimed at crippling the country's economy.

"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," Mr Shanahan said in a statement.

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.

The troop announcement came as the Pentagon released new photos which officials said provided more evidence that members of Iran's Revolutionary Guard Corps (IRGC) were responsible for last week's attacks.

The US military said the photos, taken from a Navy helicopter, showed Iranian forces removing an unexploded mine from the side of the Japanese-owned Kokuka Courageous oil tanker in the Gulf of Oman.

Other photos showed a large hole on the side of the Courageous, above the waterline, that officials said appeared to have been caused by another mine.

The US Navy says the people in this image are members of the Iranian Navy who are removing an unexploded limpet mine. ( US Department of Defence via AP )

However, the ship's owners have previously denied the ship was attacked with mines, instead saying the crew reported coming under attack from "flying objects".

The release of the photos came as the US Government works this week to convince members of Congress and allies that the accusations against Tehran are true. Iran has accused America of promoting an "Iranophobic" campaign.

Tehran, however, has repeatedly threatened to close the vital Strait of Hormuz, through which 20 per cent of the world's oil flows.

Blast damage is seen on the starboard side of the Japanese-owned Kokuka Courageous. ( US Department of Defence via AP )

The attacks on the tankers occurred alongside deteriorating relations between the US and Iran.

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

The US military has also accused the IRGC of trying but failing to shoot down a US drone, to disrupt surveillance of the tankers during the attacks.

The US says these men are members of the Iranian Navy. ( US Department of Defence via AP )

Iran said on Monday it would soon breach limits on how much enriched uranium it can stockpile under the Iran nuclear deal, which the US has withdrawn from.

A White House National Security Council spokesman said the move amounted to "nuclear blackmail".

Material left behind after what the US said was the removal of an unexploded limpet mine. ( US Department of Defence via AP )

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 has urged world powers to swiftly step up sanctions against Tehran, 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.

On Monday, US Secretary of State Mike Pompeo said the US did not want to go to war with Tehran.

Mr Pompeo said in an interview with Fox News that US President Donald Trump wanted to do "everything he can" to avoid war.

Blast damage on the Kokuka Courageous. ( US Department of Defence via A )

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