The US is sending about 1,000 more troops to the Middle East amid tensions over a series of attacks on oil tankers and with the country's nuclear agreement on the brink of collapse.
Patrick Shanahan, Acting US Defence Secretary, announced the deployment on Monday, explaining the move was for "defensive purposes," citing concerns about a threat from Iran.
The US has accused Iran of attacking six oil ships in the Gulf of Oman over the last month. Iran denies responsibility.
"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 series of mysterious attacks on oil tankers have been blamed by the US on Iranian-laid limpet mines. The Pentagon on Monday released new photos intended to bolster its case that Iran was responsible for the attacks.
Iran's paramilitary Revolutionary Guard, which the US suspects in the attacks, answers only to Supreme Leader Ayatollah Ali Khamenei and operates outside of the traditional military's control.
Gen Mohammad Hossein Bagheri, the chief of the general staff of Iran's armed forces, denied Tehran was involved in the tanker attacks, saying on Monday the country only would respond in "an open, strong and severe way" if needed.
But he also reiterated Iran's traditional stance on the Strait of Hormuz.
"If we decide to block the Strait of Hormuz, we will to do it in a way that even a drop of oil won't pass the strait," Bagheri added.
Compounding concerns in the region, the Iran nuclear agreement is in danger of collapsing by the end of the month after Iran said it would breach the agreement on June 27 unless Europe finds a way to get around US sanctions and bolster Iran’s faltering economy.
One year after Donald Trump pulled the US out of the nuclear deal, Iran said on Monday that in ten days it will blow past the limits on enriched uranium that it consented to in the 2015 agreement.
The ultimatum from Tehran is likely to trigger a diplomatic scramble by European powers to save the unravelling nuclear deal and force Britain, France, and Germany to consider whether they will sanction Iran if it openly breaks the accord.
European officials said there appeared to be little hope of reaching a compromise before the Iranian deadline. “If [the European states] had any cards, I think they would have played them by now,” one diplomat said.
If the deal does collapse it will plunge the world back into the uncertainty of the early 2010s, when Iran was amassing material that could be used for a nuclear weapon while Israel and the US threatened strikes on Iran’s nuclear facilities to stop its progress.
Under the 2015 agreement Iran was able to hold up to 300kg of low grade uranium. The country’s atomic energy agency said Monday that on June 27 it would exceed that limit, a potentially serious violation of the agreement.
"Today the countdown to pass the 300kg reserve of enriched uranium has started and in 10 days time we will pass this limit," said Behrouz Kamalvandi, a spokesman for the atomic agency.
Iran also threatened to resume enrichment of high-grade uranium, the kind that could eventually be used for a nuclear weapon. It did not set a precise date for when it would resume such enrichment, which would also breach the nuclear deal.
Hassan Rouhani, Iran’s president, said there was still time to save the nuclear agreement if Europe stepped up its support for Iran’s economy before the end of the month.
"It's a crucial moment,” Mr Rouhani told the French ambassador in Tehran. “France can still work with other signatories of the deal and play an historic role to save the deal in this very short time.”
Britain, France, and Germany have been trying to assuage Iran for months with a promise of setting up a special financial mechanism designed to let European firms trade with Iran without being hit by sanctions.
But the financial mechanism, known as Instex, has so far shown little promise and many European firms are wary of doing anything that could lead to US sanctions.
Benjamin Netanyahu, Israel’s prime minister and a leading critic of the nuclear deal, said the international community should “immediately” impose new sanctions on Iran if it carried out its threat to breach the deal.
Mr Netanyahu came close to ordering strikes on Iran’s nuclear facilities in 2011 but held off largely because of concerns that Barack Obama, then president, would not support an Israeli attack.
His calculations may be different under Mr Trump, who has been more supportive of Israel and more confrontational with Iran. Israeli officials said they were committed to stopping Iran from acquiring a nuclear weapon and would attack if necessary to achieve that objective.
The nuclear deal includes a “snapback clause” that could, if there was backing from the UN Security Council, re-impose sanctions on Iran, if it was found to have violated the agreement.
Federica Mogherini, the EU’s foreign affairs chief was asked about the EU’s red line for the nuclear deal snapback. She said the EU hoped and expected Iran to comply with its commitments.
“I will not elaborate on what happens if and when because at the moment, as of today, Iran is still compliant," she said after a meeting of EU foreign ministers in Luxembourg.
“During the last year it has been increasingly difficult for all to keep the nuclear deal fully implemented," she added,"Our focus is not to enter into a blame game. Our focus is to keep our agreement in place."
A UK spokesman said, “The E3 [of UK, France and Germany] has consistently made clear that there can be no reduction in compliance. For now Iran remains within its nuclear commitments. We are coordinating with E3 partners on next steps.”
Iran announced later on Monday that it had arrested several people in Tehran who it said were CIA operatives.
Mojtaba Zolnour, the head of Iran’s nuclear parliamentary committee, also warned that Iran could pull out of a global treaty designed to stop the spread of nuclear weapons. Mr Zolnour said that Iran could withdraw from the Non-Proliferation Treaty, under which it agreed not to seek nuclear weapons.
Iran has always claimed it has no intention of ever building a nuclear bomb but Israel and other states have accused it of secretly developing a nuclear weapons programme.