'Necessary'

Almost a day later, Iran's foreign ministry condemned the strikes. Ministry spokesperson Saeed Khatibzadeh called the airstrikes a "continuation of constant Zionist regime raids on Syrian soil," a reference to Israeli aerial attacks on Syria. Khatibzadeh also said the US troop presence in Syria was illegal and accused the country of training "terrorists."

The White House said Friday the strikes were backed by Article II of the Constitution as well as the UN charter.

"The President acted pursuant to inherent self-defense powers enshrined in our Constitution and the UN Charter," an NSC spokesperson said. "We had a rigorous process to include legal review of the strikes conducted."

The spokesperson said Biden used "his Article II authority to defend US personnel ... and to deter the risk of additional attacks over the coming weeks."

"The strikes were necessary to address the threat and proportionate to the prior attacks," the spokesman said.

The strike drew praise from one Republican and quick criticism and concern from Democrats who bristled at the White House failure to brief them.

Pennsylvania Republican Sen. Pat Toomey said Biden "is right to respond to recent Iranian-backed attacks on Americans with this airstrike."

Some Democrats complained that Biden did not respect Congress' prerogative and brief the relevant committees beforehand. While House Speaker Nancy Pelosi and Senate Majority Leader Chuck Schumer had not commented on the strikes as of Friday morning, others made their views clear.

Criticism

Sen. Tim Kaine, a Virginia Democrat, said in a statement that "offensive military action without congressional approval is not constitutional absent extraordinary circumstances. Congress must be fully briefed on this matter expeditiously."

Sen. Chris Murphy, a Connecticut Democrat, said he has "inherent trust" in Biden's national security decision-making and that the militia strikes on Iraqi bases hosting US troops are "unacceptable." But retaliatory strikes require congressional authorization, he said. "Congress should hold this administration to the same standard it did prior administrations, and require clear legal justifications for military action, especially inside theaters like Syria, where Congress has not explicitly authorized any American military action," said Murphy.

Rep. Ro Khanna, a California Democrat on the House Armed Services Committee, told CNN, "there is absolutely no justification for a president to authorize a military strike that is not in self-defense against an imminent threat without congressional authorization. We need to extricate from the Middle East, not escalate."

"The President should not be taking these actions without seeking explicit authorization," said Khanna, who noted that Biden is now "the fifth consecutive US president to order strikes in the Middle East."

"I spoke against endless war with Trump, and I will speak out against it when we have a Democratic President," Khanna added.

The site the US struck is believed to be part of a weapons smuggling operation by the militias, according to a US official. The strikes were conducted to degrade the groups' ability to carry out attacks and to send a message about the recent attacks, the official said.

The site was used by Iraqi Hezbollah militias near al-Hurri village right on the Syrian-Iraqi borders inside Syria, a resident in Albu Kamal city told CNN under the condition of anonymity for security reasons.

Kata'ib Hezbollah claimed in a brief statement Friday that one of its fighters had been killed. It statement said the killed fighter "was stationed on the Iraqi-Syrian border to protect the land of Iraq and its people from ISIS criminal gangs."

The US military's original plan was to strike two sites near the Syrian city of Albu Kemal, but at the last minute, the second location was scratched because of concerns that civilians might be in the area, a US defense official tells CNN.

Decision made from the 'top down'

The decision to target the site in Syria was made from the "top down," a defense official said. Austin told reporters that Biden had authorized the strike on Thursday morning, after he had recommended the President take action.

"We're confident in the target we went after," Austin said on a flight back to Washington from San Diego on Thursday. "We know what we hit. We allowed and encouraged the Iraqis to investigate and develop intelligence, and that was very helpful to us in refining the target."

Kirby said Biden authorized the strikes after consulting with US allies, including coalition partners, and that they had taken place at about 6 p.m. ET. The Russian military was given a warning about the upcoming airstrike "four or five minutes" before it was delivered, Russian Foreign Minister Sergey Lavrov said Friday, according to the Russian state news agency TASS.

The airstrikes were conducted by two F15s dropping Joint Direct Attack Munitions. A JDAM is a precision weapon with GPS navigation capabilities.

While the US had not before Thursday blamed any specific group for the rocket attacks or attributed them to any Iranian proxies in the region, the administration has made clear where it places the ultimate blame.

"We have stated before that we will hold Iran responsible for the actions of its proxies that attack Americans," State Department spokesman Ned Price said on Monday, noting that "many of these attacks have used Iranian-made, Iranian-supplied weapons."

Earlier this week, White House spokeswoman Jen Psaki had said the US holds Iran accountable for the actions of its proxies.

A February 15 rocket attack on coalition forces near the Erbil International Airport in Iraqi Kurdistan killed a civilian contractor and injured nine others, including four American contractors and one member of the US military who went through concussion protocol. That day, about 14 rockets were fired toward US and coalition forces in Erbil, in northern Iraq. At the time, Psaki said Biden and his administration "reserve the right to respond in a manner and at a time of our choosing."

She warned that "we will respond in a way that's calculated on our timetable, and using a mix of tools, seen and unseen."

"What we will not do, and what we've seen in the past, is lash out and risk an escalation that plays into the hands of Iran by further destabilizing Iraq, and that is our priority," Psaki added.

That attack was the first of three that came in rapid succession.

Over the weekend, at least four rockets struck Balad Air Base north of Baghdad, where a US defense company works on Iraqi combat aircraft.

Then on Monday, two rockets landed in Bahgdad's international zone, where many foreign embassies are located. There were no reports of injuries or damage.

The US strikes come as Washington and Tehran position themselves for negotiations about Iran's nuclear program, potentially complicating an already fragile process.

Iran Foreign Ministry spokesman Saeed Khatibzadeh denied any ties to the February 15 attack in Erbil, and Iran has not claimed responsibility for any of the other strikes. "While these rumors are strongly rejected, the dubious attempt to attribute it to Iran is also strongly condemned," Khatibzadeh said, according to a February 16 report by Iran's state official news agency Mehr.

This story has been updated with more details and background.

CORRECTION: An earlier version of this story misstated the committee Rep. Ro Khanna serves on. He is a member of the House Armed Services Committee.