WASHINGTON — Tensions between the United States and Iran flared on Monday as Tehran said it would soon breach a key element of the 2015 international pact limiting its nuclear program, while President Trump ordered another 1,000 troops to the Middle East and vowed again that Iran would not be allowed to develop a nuclear weapon.
The Pentagon’s announcement of the troop deployment came three days after attacks on two oil tankers in the Gulf of Oman that the administration has blamed Iran for. And it came hours after Iran said it was within days of violating a central element of the landmark 2015 agreement — intended to curb its ability to develop a nuclear weapon — unless European nations agreed to help it blunt crippling American economic sanctions.
Iran’s Atomic Energy Organization said that within 10 days the country will have produced and kept in its stockpiles more low-enriched uranium — the sort used to fuel power plants — than allowed by the 2015 containment deal. The agency also left open the possibility that it might soon begin enriching the uranium to higher levels of purity, edging it closer to what would be necessary to build a nuclear weapon.
Mr. Trump pulled out of the 2015 pact last year, saying that it was not tough enough on Iran. In doing so, he put intense strain on the international coalition that had backed the agreement and wanted to keep it alive. And he left Iran trapped between continuing to abide by the deal’s provisions without getting any of its benefits or abandoning it and provoking a more intense conflict with the United States.
With Iran now on the verge of breaching the deal, the White House called for greater international pressure on the country, even as European officials urged restraint between the two longtime adversaries.
“President Trump has made it clear he will never allow Iran to develop nuclear weapons,” the National Security Council said in a statement.
The additional 1,000 troops being sent to the region comes on top of 1,500 dispatched in May. They will be used mainly for surveillance of Iranian activities and protecting American forces already in the Middle East. The Pentagon had considered plans for deployment of up to 6,000 additional troops.
“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,” the acting defense secretary, Patrick Shanahan, said in a statement.
The announcement from Tehran was Iran’s latest signal that it would abandon the 2015 pact unless other signatories help it offset economic sanctions imposed by Mr. Trump. The threat seemed aimed primarily at European countries to persuade them to break with Washington and restore to Tehran some of the economic benefits of the deal.
Iran had been abiding by the terms of the nuclear deal, negotiated under President Barack Obama, before Mr. Trump pulled out, and has continued to do so since the withdrawal by the United States. But as American sanctions have squeezed the Iranian economy, Tehran has warned that it could not remain in the deal without getting European help to find workarounds to the sanctions.
“This was an entirely predictable consequence of the Trump administration’s withdrawal from the nuclear deal and maximum pressure strategy,” said Ali Vaez, the director of the Iran project at the International Crisis Group, a conflict-resolution organization. “In practice, maximum pressure has produced maximum peril and minimum strategic results.”
[The U.S. has turned up the pressure on Iran. See the timeline of events.]
The mechanism of American sanctions may actually have sped Iran to the point where its stockpile of uranium is on the verge of violating the 2015 agreement’s terms. In May, the State Department announced that it might penalize countries that transfer enriched uranium out of Iran.
Until now, Iran has shipped most of the low-enriched uranium it produces out of the country, swapping it for natural uranium. That allows it to continue producing small amounts of nuclear fuel for civilian power plants without building up a stockpile for potential use in weapons.
During a news conference announcing Tehran’s decision, Behrouz Kamalvandi, a spokesman for Iran’s Atomic Energy Organization, said Iran might also enrich its uranium up to 20 percent purity for use in reactors, the Iranian state-run news organization Press TV reported.
He said that uranium would be used as fuel for the Tehran Research Reactor, which the United States supplied to Iran in 1967 and that Iranian officials say is used to create medical isotopes for use in cancer treatment.
The nuclear agreement limits the level of enrichment to 3.67 percent, but if Iran began producing 20 percent enriched uranium, it would put the country much closer to weapons-grade levels.
Members of Congress braced themselves for a potential fight with Mr. Trump over authorization for military action.
“Now we are stumbling to the brink of war without the support of our allies,” said Senator Tom Udall, Democrat of New Mexico and a member of the Foreign Relations Committee. “Congress must step up and prevent an unconstitutional war with Iran and avert one of the biggest foreign policy blunders in decades.”
Over the past year, the Trump administration imposed severe economic sanctions that have discouraged most outside companies from doing business with Iran, and followed that up with measures to cut off Iran’s oil revenues, the lifeblood of its economy. The sanctions have had a great effect on Iran, including leading to a shortage of critical medicine, despite Secretary of State Mike Pompeo’s assertions that humanitarian aid would not be affected.
Recent attacks on oil tankers in the Strait of Hormuz, which the Trump administration has blamed on Iran, have further inflamed matters. Mr. Pompeo plans to travel on Tuesday to Central Command in Florida to discuss Middle East security with commanders.
The Pentagon released additional pictures on Monday that it said bolstered its case that Iran was responsible for the attack on the tankers last week. Tehran has denied responsibility.
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Defense Department officials said one of the images shows sailors with the Revolutionary Guards removing an unexploded limpet mine from one of the tankers, the Kokuka Courageous, in the hours after an initial explosion. Another photograph, the Pentagon said, shows the “remnants of the magnetic attachment device of unexploded limpet mine” placed on one of the tankers.
The officials said the limpet mines were placed above the water line of the ships, where they would be visible but would do relatively limited damage, and not below the water line, where they could actually cause the ships to take on water. One official, speaking on the condition of anonymity to discuss intelligence issues, said that the Pentagon interpreted this as Iran trying to send a message of what its abilities are in the gulf, without doing real damage to shipping.
The official described it as a “nuisance attack.”
A National Security Council spokesman, Garrett Marquis, said Monday that Iran’s announcement on its uranium was “nuclear blackmail” that “must be met with increased international pressure.”
The White House national security adviser, John R. Bolton, has been a longtime advocate for regime change in Iran.
China and Russia were both signatories to the 2015 nuclear agreement and have opposed Mr. Trump’s Iran policies. Beijing has said it intends to continue buying oil from Iran despite American sanctions.
Germany, Britain and France have worked to set up a system to allow European companies to take part in a kind of barter trade with Iran.
On Sunday, Helga Schmid, a senior European Union diplomat, visited Tehran for meetings on the nuclear deal.
The Netherlands’ foreign minister, Stef Blok, said Monday that European support for the nuclear deal depended on Iran adhering to the pact’s terms.
“As long as Iran is fulfilling these criteria,” he said, “we should stick to this deal.”
Iranian officials on Monday also taunted Washington over the exposure of a C.I.A. informant network in Iran years ago. Ali Shamkhani, the secretary of Iran’s Supreme National Security Council, said that “one of the most complicated C.I.A. cyberespionage networks that had an important role in the C.I.A.’s operations in different countries was exposed by the Iranian intelligence agencies” and was “dismantled,” according to state-run broadcast network IRIB. He added that the Iranians shared their information with “allies,” which led to arrests of C.I.A. “agents.”
Mr. Shamkhani appeared to be referring to an operation that broke into the C.I.A.’s covert communications system, or “covcom,” and uncovered informants in Iran around 2012 and 2013. That operation may also have contributed to the extraordinary crippling of the C.I.A. informant network in China. Iranian officials made a similar claim in April, and a former C.I.A. official said they may be reviving the taunt now to try to portray the United States as an aggressor.