The US and Iran edged toward a flashpoint on Monday as Tehran announced it was breaking compliance with the accord that keeps it from making nuclear weapons and the Trump administration followed by ordering 1,000 more troops to the Middle East.
The Pentagon said the deployment includes security forces and troops for additional surveillance and intelligence gathering in the region.
While the number is small, it represents an escalation of US military might aimed at deterring Iran and calming allies worried that transit through key shipping lanes could be in jeopardy.
Tehran's announcement earlier Monday means it could soon start to enrich uranium to just a step away from weapons-grade levels, challenging President Donald Trump's assurances to allies that the US withdrawal from the deal last year made the world a safer place.
The developments are bound to inflame tensions in the Middle East and pose a test of resolve and credibility for both adversaries.
Iran said it would break a limit on uranium stockpiles established by the 2015 agreement with world powers that was intended to restrict the Islamic Republic's nuclear program in exchange for an easing of international sanctions.
After Trump withdrew from the agreement, signed by his predecessor, he reinstated punishing economic sanctions, leaving the European and other partners in the accord struggling to keep Iran on board.
US' awkward position on nuclear deal
On Monday, the US administration found itself in the awkward position of demanding that Iran comply with a nuclear accord that the president derided as the worst deal in history.
"We continue to call on the Iranian regime not to obtain a nuclear weapon, to abide by their commitments to the international community," State Department spokeswoman Morgan Ortagus told reporters.
The move comes as Washington accuses Iran of attacking two tankers near the Persian Gulf and the Iranians deny responsibility. With details murky and no one owning up to the attacks, the Pentagon released new photos intended to bolster its case that Iran carried out the attacks.
The State Department spokeswoman said Iran's uranium announcement amounted to "extortion" and a "challenge to international norms," as well as to the 2015 agreement known formally as the Joint Comprehensive Plan of Action.
"It's unfortunate that they have made this announcement today," Ortagus said.
"It doesn't surprise anybody and this is why the president has often said that the JCPOA needs to be replaced with a better deal."
Trump appeared to say the deal should not be violated in a tweet: "Iran to defy Uranium Stockpile Limits."
In announcing the new deployment, Acting Defense Secretary Patrick Shanahan said the forces are "for defensive purposes to address air, naval, and ground-based threats in the Middle East."
"The United States does not seek conflict with Iran," Shanahan said. "The action today is being taken to ensure the safety and welfare of our military personnel working throughout the region and to protect our national interests."
He added that the US will continue to adjust troop levels as needed.
On the unravelling of the multinational nuclear deal, some of its supporters blamed the Trump administration for Iran's provocative announcements, saying they were predictable given the renewed US pressure.
"While Iran's frustration with Trump's reckless and irresponsible pressure campaign is understandable, we strongly urge Iran to remain in compliance with the nuclear deal," the Arms Control Association said in a statement.
"It remains in Iran's interests to abide by the limits of the agreement and to fully cooperate with the International Atomic Energy Agency's more intrusive monitoring and verification."
Iran has shown no willingness to negotiate another deal and vowed not to enter into talks with the United States while the administration maintains its "maximum pressure" campaign of sanctions.
Administration officials found themselves Monday grappling with whether to press the remaining parties to the deal, including Britain, France, and Germany, to demand that Iran stay in compliance.
They must also consider if such a stance would essentially concede that the restrictions imposed during the Obama administration, while short of ideal, are better than none.
Under the deal, Iran can keep a stockpile of no more than 300 kilograms of low-enriched uranium. Behrouz Kamalvandi, spokesman for Iran's atomic agency, said it would pass that limit June 27.
A senior US official said the administration is most concerned about any violation of the deal that would reduce the breakout time that Iran would need to produce a nuclear weapon.
The deal aimed to keep the breakout time at one year.
The official said certain violations, while they should be not accepted, would not necessarily reduce that time. But other violations, such as enriching uranium to 20 percent, should be addressed immediately if they occur, the official said. The official was not authorised to discuss the matter publicly and spoke on condition of anonymity.
The official said it would be up to the Europeans to decide if Iran was in violation of the deal and whether to initiate a dispute resolution mechanism that could bring the Iranians back into compliance.
Secretary of State Mike Pompeo is expected to meet this week with EU Foreign Policy Chief Federica Mogherini, a leading deal proponent, at which this issue is likely to be raised.
Pompeo, who was a leading critic of the deal while he was in Congress, has said in the past that Iranian compliance is not really an issue as the administration sees the agreement as fundamentally flawed because over time it eases many limits on Iran's nuclear activities.
Yet, just last week, the US ambassador to the UN's nuclear watchdog accused Iran of violating a provision of the deal that relates to advanced centrifuges and called on the Europeans to ensure that Iran remains in compliance.