Iran retaliated for the killing of Gen. Qassem Soleimani by firing more than a dozen ballistic missiles at two Iraqi air bases housing U.S. forces on Wednesday local time.
Washington and Tehran both confirmed that Iran was the source of the missiles. The extent the damage was unclear.
Iran's Supreme Leader Ayatollah Ali Khamenei said the strikes were a "slap in the face" to the U.S. and not sufficient retaliation for the killing of Soleimani, a top general, last week.
In a White House address, President Donald Trump said: "No Americans were harmed in last night's attack by the Iranian regime. We suffered no casualties. Our great American forces are prepared for anything. Iran appears to be standing down."
He had earlier tweeted "all is well!" and "so far, so good!"
The attacks at 1:20 a.m. local time (5:30 p.m. ET Tuesday) came after Iranian leaders had promised "revenge" and "harsh retaliation" for Soleimani's death in a U.S. airstrike near Baghdad airport.
A spokesman for Iran's powerful Revolutionary Guard said the timing of Wednesday's strikes was symbolic, coming at roughly the same time that Soleimani died, Iran's state-run IRNA news agency reported.
Iranian Foreign Minister Mohammed Javad Zarif said Wednesday his country did not "seek escalation or war, but will defend ourselves against any aggression."
The missiles targeted at least two Iraqi military bases hosting U.S. and coalition forces, the Department of Defense said in a statement.
One of these was the Ain al-Asad air base, which is around 100 miles northwest of Baghdad and which Trump visited in 2018. The Pentagon did not directly name the second base but said it was in or around Irbil, in the Kurdish-run north of the country.
NATO said in an emailed statement that there were no causalities among its mission in Iraq, which comprises about 500 trainers, advisers and supporting personnel from the U.S. and elsewhere. On Tuesday, NATO Secretary-General Jens Stoltenberg said the alliance had suspended training operations in the country.
U.S. forces at Iraqi bases most likely had some warning before the missiles struck, thanks to a facility devoted to detecting and providing alerts about launches anywhere in the world, according to public documents and a former senior intelligence official.
Iraqi Prime Minister Adel Abdul Mahdi said in a statement he was warned by Iran shortly before midnight that the attack "had begun or would begin shortly."
Mahdi said he was told "the strike would be limited to the locations of the U.S. military in Iraq" and he "alerted the Iraqi forces and military commanders to take necessary precautions."
Mahdi said he had not received any reports of casualties among the Iraqi or U.S.-led coalition forces.
On Monday, Secretary of State Mike Pompeo reiterated that classified U.S. intelligence showed Soleimani was involved in planning attacks on Americans in the region.
The U.S. blames Soleimani for a spate of rocket attacks in recent months, including one that killed a U.S. contractor in late December.
Soleimani was one of the most influential figures in the Middle East, having developed a network of powerful militia groups whose clandestine reach stretched into Iraq, Syria and beyond.
The attacks are the latest development in the United States' rapidly deteriorating relations with Iran. Tensions began simmering after Trump unilaterally withdrew from a nuclear deal between Iran and several world powers in May 2018.
European allies have tried to salvage the deal, which restricted nuclear development in exchange for the easing of crippling economic sanctions. The agreement limited Tehran's uranium enrichment and the amount of enriched uranium it could stockpile, as well as its nuclear research and development.
Iran announced Sunday that it would no longer abide by the agreement and that recent events meant it would take an even bigger step away from the deal than it had initially planned, with no further limits on uranium enrichment.
Zarif said there "will no longer be any restriction on number of centrifuges."
Trump's decision to target Soleimani was met with mixed reactions, as some feared that the general's death would lead to another war in the Middle East.
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Trump warned Saturday that the United States had 52 potential targets "very important to Iran & the Iranian culture" if Iran planned a retaliatory attack. The figure symbolizes the number of hostages held by Iran in 1979, when 52 U.S. diplomats and citizens were seized and held for 444 days.
House Speaker Nancy Pelosi, D-Calif., tweeted Tuesday that the U.S. and the world "cannot afford war."
"Closely monitoring the situation following bombings targeting U.S. troops in Iraq. We must ensure the safety of our service members, including ending needless provocations from the Administration and demanding that Iran cease its violence. America & world cannot afford war," she said.