U.S.-Iran tensions run high after anniversary of Soleimani's death

American officials said they fear a strike could be more significant than the periodic rocket attacks that Iran-linked militias in Iraq have lobbed at bases where U.S. troops are located or at the U.S. Embassy in Baghdad, and that the militiamen in Iraq have added new advanced weaponry to their arsenals.

"We still believe that that could rapidly move from planning to execution with little or no notice," a U.S. official said, speaking on the condition of anonymity to describe an assessment of Iran's intentions. The concern, the official continued, is that any potential action "is not going to look like the standard proxy attack."

The officials did not provide evidence of the Iranian preparations or say what led them to conclude that Tehran was transferring weapons into Iraq. Iran's foreign minister has warned in recent days that "provocateurs" may be planning an attack on U.S. interests to bait the United States into a war in the final days of the Trump administration.

The U.S. assessment comes as Iran took a major step away from the 2015 international nuclear deal. According to Ali Rabie, a government spokesman, Tehran notified the International Atomic Energy Agency on Monday that it had begun 20% uranium enrichment at Fordow, an underground facility near the city of Qom.

The action defies the terms of the landmark agreement with world powers, which restricts Iranian enrichment and makes the Fordow site off-limits for uranium.

Iran began increasing its nuclear activities after Trump withdrew from the deal in 2018 as part of his "maximum pressure" campaign against Tehran, which it has identified as its chief rival in the Middle East.

The Jan. 3 strike on Soleimani, who oversaw a network of Iranian-supported proxy groups across the Middle East, marked the apex of that extended confrontation with Iran. Days later, Iran launched a significant missile attack on a U.S.-occupied base in Iraq, injuring scores of U.S. troops.

In recent weeks, Iranian officials have warned of further retaliation and issued threats against the United States, saying that not even President Donald Trump is safe.

In another apparent sign of intensifying concern, acting Defense Secretary Christopher Miller late Sunday abruptly reversed last week's decision to send the aircraft carrier USS Nimitz back to the United States from the Middle East. The carrier, which transports fighter jets and electronic attack aircraft and is accompanied by a flotilla including guided-missile destroyers, is now located in the Far Eastern section of U.S. Central Command's naval zone, near India.

The nuclear-powered Nimitz, the Navy's oldest functioning carrier, has been part of the response to earlier U.S. confrontations with Iran. In 1979, U.S. pilots launched aircraft from her decks in an attempt to rescue Americans held hostage in Tehran. Before traveling east, the Nimitz supported an operation to reduce the U.S. troop footprint in Somalia.

Miller's decision on the Nimitz is the latest in a series of dramatic decisions during the final months under Trump, who fired Miller's predecessor Mark Esper after the Nov. 3 election.

In the lead-up to the first anniversary of the strike on Soleimani, which also killed a senior Iraqi militia figure, the Pentagon has taken other steps intended to deter Iran, including flying B-52 bombers to the region, and has reduced the staff at the U.S. Embassy in Baghdad. There are about 2,500 U.S. troops in Iraq.

Iran-linked militia groups have been blamed for attacks on U.S. diplomatic and military facilities in Iraq over the past year, most recently firing a barrage of 21 rockets at the U.S. Embassy after a months-long lull in hostilities. But in the run-up to Trump's departure from office, Iraqi militia officials have mostly appeared to want to rein in any possibility of escalation, condemning rocket attacks and insisting that they do not intend to threaten the U.S. Embassy in the short term.

"We will not enter the embassy of evil nor topple the government, there is plenty of time for that," Abu Hussain al-Hamidawi, secretary general of Iraq's Kataib Hezbollah militia, said Sunday in a statement.

Thousands of Iraqi militia supporters gathered in Baghdad on Sunday to commemorate the deaths in a vociferous but tightly managed event. From a stage above the central Tahrir Square, militia officials urged the expulsion of U.S. troops from Iraq as the crowd chanted anti-American songs.

The Soleimani strike intensified a long-standing dilemma for Iraqi leaders, who juggle the desires of the United States, a major financial and military ally, with those of Iran, with whom Iraq shares a long border and deep religious and social ties.

The nuclear deal allows Iran to enrich uranium to a 3.67% concentration of uranium-235, a fissile isotope, at another site and to maintain a small stockpile of it to use as fuel for its nuclear power reactors. Uranium enriched to 20% U-235 is suitable for use in an old, U.S.-supplied research reactor in Tehran that began operating in 1967.

The 20% enrichment level is a relatively short, technical step from the 90% needed for the fissile material in a nuclear weapon. Iran began increasing its nuclear activities after the Trump administration withdrew from the agreement, which curbed Tehran's nuclear program in exchange for major sanctions relief.

The United States then began reimposing major sanctions. In response, Iran said it would progressively abandon some elements of the deal, notably the limits on the purity and size of its enriched-uranium stockpile, though it has maintained its commitment in the deal that it will not build or acquire nuclear arms.

The IAEA said in a statement Monday that it has informed member states that Iran "began feeding uranium already enriched up to 4.1 percent U-235 into six centrifuge cascades at Fordow for further enrichment up to 20 percent." It said "IAEA inspectors were present at the site" for the start of the process.

Iran's enrichment announcement, just two weeks before President-elect Joe Biden is set to be sworn in, may constitute a new obstacle to his team's stated goal of rejoining the nuclear deal if Iran also returned to compliance under the agreement.

Israel, which maintains that Tehran is seeking nuclear weapons, immediately condemned the Iranian move. Prime Minister Benjamin Netanyahu tweeted that it "cannot be explained in any way other than the further realization of its intention to develop a military nuclear program."

Iran, meanwhile, has denounced recent shows of force as provocative and has suggested that Israel may take action of its own. On Saturday, Iranian Foreign Minister Mohammad Javad Zarif wrote on Twitter that "new intelligence from Iraq indicate that agent-provocateurs are plotting attacks against Americans - putting an outgoing Trump in a bind with a fake casus belli," a term for an action that justifies a war.

Norman Roule, who served as the top U.S. intelligence officer on Iran, said the new enrichment was an attempt to build leverage with the Biden administration and the European Union, and to show defiance toward the Trump administration. "This step is reversible," he wrote on Twitter.