Acting US Defense Secretary Patrick Shanahan announced on Monday the deployment of about 1,000 more troops to the Middle East for what he said were defensive purposes, citing 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 in the Gulf of Oman, which Washington has blamed on Tehran, more than a year after US President Donald Trump said Washington was withdrawing from a 2015 nuclear deal.
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"The recent Iranian attacks validate the reliable, credible intelligence we have received on hostile behavior by Iranian forces and their proxy groups that threaten United States personnel and interests across the region," 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. Washington previously tightened sanctions, ordering all countries and companies to halt imports of Iranian oil or be banished from the global financial system.
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.
The US military released additional imagery on Monday.
"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 defense 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. The deployment would also involve personnel who can strengthen the protection of US troops already deployed to the region.
Iran's armed forces chief of staff, Maj. Gen. Mohammad Baqeri, denied on Monday that Tehran was behind the tanker attacks and said if the Islamic Republic decided to block the strategic Strait of Hormuz shipping lane, it would do so publicly.
The secretary of Iran's Supreme National Security Council, Ali Shamkhani, said Tehran was responsible for security in the Gulf and urged US forces to leave the region, state TV said.
US Secretary of State Mike Pompeo has spoken to officials from the North Atlantic Treaty Organization, China, Kuwait, South Korea, Britain and other countries to share evidence of Iran's involvement in the attacks on the Norwegian and Japanese tankers, a senior State Department official said.
Pompeo said on Sunday the United States did not want to go to war with Iran but would take every action necessary, including diplomacy, to guarantee safe navigation through Middle East shipping lanes.
Iran also said on Monday 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 Trump's decision, caps Iran's stock of low-enriched uranium at 300 kilograms enriched to 3.67%.
But Iran's Atomic Energy Organization spokesman Behrouz Kamalvandi said on Monday: "We have quadrupled the rate of enrichment [of uranium] and even increased it more recently, so that in 10 days it will bypass the 300-kilogram 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 fulfill 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, Iran's arch foe, urged world powers to step up sanctions against Tehran 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.
