WASHINGTON — President Joe Biden on Thursday ordered airstrikes on buildings in Syria that the Pentagon said were used by Iranian-backed militias, in retaliation for rocket attacks on U.S. targets in neighboring Iraq.

Pentagon press secretary John Kirby portrayed the bombing in eastern Syria as carefully calibrated, calling it “proportionate” and “defensive.”

The operation was the first known use of military force by the Biden administration, which has for weeks emphasized plans to focus more on challenges posed by China.

The president’s decision appeared aimed at sending a signal to Iran and its proxies in the region that Washington would not tolerate attacks on its personnel in Iraq, even at a sensitive diplomatic moment.

Three rocket attacks in one week in Iraq, including a deadly strike that hit a U.S.-led coalition base in the northern Iraqi town of Irbil, presented a test for Biden only weeks after assuming the presidency. The rocket assaults coincided with a diplomatic initiative launched by the administration to try to revive a 2015 nuclear agreement between Iran and world powers.

The airstrikes “were authorized in response to recent attacks against American and coalition personnel in Iraq, and to ongoing threats to those personnel,” Kirby said in a statement.

The operation “destroyed multiple facilities located at a border control point used by a number of Iranian-backed militant groups,” including Kataib Hezbollah and Kataib Sayyid al-Shuhada, he said.

Syrian and Iranian officials did not immediately react to the strikes.

The Syrian Observatory for Human Rights said Friday that 22 people were killed in the strikes. The London-based monitoring group did not provide any details about how it obtained that figure. Iran’s state broadcaster IRIB news, meanwhile, said 17 “resistance fighters” were killed in the strikes, but also didn’t provide detail about the source of that figure other than citing “reports.”

A senior U.S. defense official told NBC News on Thursday evening that the target was a transit hub near the Iraqi-Syrian border used by the militia fighters, and it was too early to say what casualties might have been inflicted on the militants.

“The operation sends an unambiguous message: President Biden will act to protect American and coalition personnel. At the same time, we have acted in a deliberate manner that aims to de-escalate the overall situation in both eastern Syria and Iraq,” he said.

Two U.S. aircraft were involved in the strikes that took place at about 6 p.m. EST on Thursday, or 2 a.m. Friday in Syria, the official said.

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Defense Secretary Lloyd Austin told reporters travelling with him that the administration had been “very deliberate about our approach.”

“We’re confident that target was being used by the same Shia militia that conducted the strikes,” Austin said, referring to the recent rocket attacks in Iraq on U.S. and coalition personnel.

The Pentagon had said previously that it was awaiting the results of an Iraqi investigation into the Irbil rocket attack.

“We allowed and encouraged the Iraqis to investigate and develop intelligence and that was very helpful to us in refining the target,” said Austin, who spoke en route to Washington after a visit to California and Colorado.

Biden had approved the operation on Thursday morning, he said.

A civilian contractor was killed in the Irbil rocket assault, and a U.S. service member and others were wounded. At least two 107mm rockets landed on the base, which also hosts Irbil’s civilian international airport.

NBC News had previously reported that Iranian-backed militias were most likely behind the Irbil rocket attack, and that the weapons and tactics resembled previous attacks by the Iranian-linked militias. However, it was unclear if Iran had encouraged or ordered the rocket attack.

An obscure group called Saraya Awliya al-Dam, or Custodians of the Blood, claimed responsibility for the Irbil attack. But former diplomats and regional analysts said the group was merely a front organization created by the main Shiite militias in Iraq.

Following the rocket attack on the Irbil base, Iraq’s Balad air base came under rocket fire days later, where a U.S. defense firm services the country’s fighter jets, and then two rockets landed near the U.S. Embassy compound in Baghdad.

Iran has rejected any connection to the rocket attacks.

In a phone call Tuesday between Biden and Iraqi Prime Minister Mustafa al-Kadhimi, the two leaders agreed that “that those responsible for such attacks must be held fully to account,” according to a White House readout of the conversation.

Dennis Ross, a former senior U.S. diplomat who worked on Middle East policy under several presidents, said the administration had lowered the risk of causing friction with the Iraqi government by hitting targets in Syria.

“By striking facilities used by the militias just across the border in Syria, the risk of blowback against the Iraqi gov is reduced,” Ross tweeted.

Dan De Luce and Mosheh Gains reported from Washington; Ali Arouzi reported from London; Amin Hossein Khodadadi reported from Tehran; and Charlene Gubash reported from Cairo.

The Associated Press contributed.

Ali Arouzi, Amin Hossein Khodadadi and Charlene Gubash contributed.