WASHINGTON — The United States carried out airstrikes early Monday morning in Iraq and Syria against two Iranian-backed militias that the Pentagon said had conducted drone strikes against American personnel in Iraq in recent weeks, the Defense Department said.
“At President Biden’s direction, U.S. military forces earlier this evening conducted defensive precision airstrikes against facilities used by Iran-backed militia groups in the Iraq-Syria border region,” the Pentagon spokesman, John F. Kirby, said in a statement.
Mr. Kirby said the facilities were used by Iranian-backed militias, including Kata’ib Hezbollah and Kata’ib Sayyid al-Shuhada, to store arms and ammunition for carrying out attacks against places where Americans were located in Iraq. There were no immediate reports of casualties but a military after-action review is ongoing, Pentagon officials said.
The strikes were the second time that Mr. Biden has ordered the use of force in the region. The United States carried out airstrikes in eastern Syria in late February against buildings belonging to what the Pentagon said were Iran-backed militias responsible for attacks against American and allied personnel in Iraq.
The latest strikes were carried out by U.S. Air Force fighter-bombers based in the region.
Pentagon planners have been gathering information for weeks on the sites and militia networks that use them, American officials said on Sunday. Defense Secretary Lloyd J. Austin III and Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, briefed Mr. Biden on attack options early last week, and Mr. Biden approved striking the three targets, the officials said.
The strikes were carried out a little more than a week after Iran elected a hard-liner, Ebrahim Raisi, as its next president.
The military action also came as the negotiations intended to bring the United States and Tehran back into compliance with an international nuclear accord have reached a crucial juncture. President Donald J. Trump pulled the United States out of the accord in 2018, and Mr. Biden has been seeking to revive it.
On Sunday, Secretary of State Antony J. Blinken discussed the negotiations on the nuclear deal with Israel’s foreign minister, Yair Lapid, who said Israel had “serious reservations” about the accord, which would ease sanctions on Iran in return for limits on its nuclear program.
Earlier this month, the Biden administration blocked access to myriad websites linked to Iran after the nation held a presidential vote to install Mr. Raisi, a close ally of the clerical government’s supreme leader, as its top elected official.
Pressure has been building for weeks from Democrats and Republicans in Congress, and from some of Mr. Biden’s top advisers and commanders, to retaliate against the threat posed by the drones to American diplomats and the 2,500 U.S. troops in Iraq who are training and advising Iraqi forces.
At least five times since April, the Iranian-backed militias have used small, explosive-laden drones that divebomb and crash into their targets in late-night attacks on Iraqi bases — including those used by the C.I.A. and U.S. Special Operations units, according to American officials. So far, no Americans have been hurt in the attacks, but officials worry about the precision of the drones, also called unmanned aerial vehicles, or U.A.V.s.
The drones are part of a rapidly evolving threat from Iranian proxies in Iraq, with militia forces specialized in operating more sophisticated weaponry hitting some of the most sensitive American targets in attacks that evaded U.S. defenses.
Iran — weakened by years of harsh economic sanctions — is using its proxy militias in Iraq to step up pressure on the United States and other world powers to negotiate an easing of those sanctions as part of a possible revival of the 2015 nuclear deal. Iraqi and American officials say Iran has devised the drone attacks to minimize casualties to avoid prompting U.S. retaliation.
American officials said the strikes — against two targets in eastern Syria and a third just across the border in Iraq — were carried out about 1 a.m. Monday local time by a mix of Air Force F-16s and F-15Es based in the region.
The fighter-bombers dropped multiple bombs — 500-pound and 2,000-pound satellite-guided munitions — on each of the three structures. American officials said the militias used the sites targeted in Syria mainly for storage and logistics purposes; the site hit in Iraq was used to launch and recover the armed drones, which officials said were either made in Iran or used Iranian technology.
Mr. Kirby and other administration officials characterized the strikes as defensive, but leading lawmakers demanded more details on Sunday.
“Congress must be briefed on these airstrikes without delay,” said Senator Tim Kaine, Democrat of Virginia, who has led the fight to limit presidential war powers for a decade from the Senate Foreign Relations Committee. “If the strikes were against militias that were using U.A.V.s to attack American personnel, that would be a conventional self-defense action that is justified. But we need to know more.”
Michael P. Mulroy, a former C.I.A. officer and top Middle East policy official at the Pentagon, has warned that with the technology provided by Iran’s Quds Force — the foreign-facing arm of Iran’s security apparatus — the drones are rapidly becoming more sophisticated at a relatively low cost.
“This action should send a message to Iran that it cannot hide behind its proxy forces to attack the United States and our Iraqi partners,” Mr. Mulroy said on Sunday.
But Mr. Biden’s top aides have also said they want to avoid the angry rhetorical jabs and tit-for-tat threats that Mr. Trump often engaged in with Iran and its proxies in Iraq, and avoid escalating tensions with Tehran at a time when the White House is trying to nail down the nuclear deal.
The airstrikes in February against the same militias were also a relatively small, carefully calibrated military response: seven 500-pound bombs dropped on a small cluster of buildings at an unofficial crossing at the Syria-Iraq border used to smuggle across weapons and fighters.
Those earlier strikes were just over the border in Syria to avoid diplomatic blowback to the Iraqi government. The same calculus influenced the planning for the strike on Monday — two of the three targets were in Syria along the Iraqi border, and the third was just inside Iraqi territory. The strikes took place early Monday in part to avoid any civilian casualties, officials said.
“The United States took necessary, appropriate and deliberate action designed to limit the risk of escalation — but also to send a clear and unambiguous deterrent message,” Mr. Kirby said.
How the militias and Iran will respond is unclear, and American officials said the relatively small set of airstrikes were unlikely to stop the militia attacks completely. After the February strikes, there was a lull in militia activity against American locations for several weeks, but then an even more dangerous threat emerged: the small armed drones.
Jennifer Steinhauer, Julian Barnes and John Ismay contributed reporting.