U.S. military forces had launched airstrikes in Iraq and Syria on Sunday, June 27, targeting militia groups behind the drone attacks on American personnel.

A Pentagon official called the airstrikes a defensive one, NBC News reported. In a statement, Pentagon Press Secretary John Kirby said the airstrikes focused on operational and weapons storage facilities at two locations in Syria and one in Iraq.

The said facilities were utilized by at least two militias identified as Kata'ib Hezbollah and Kata'ib Sayyid al-Shuhada. Kirby noted that President Joe Biden had authorized the airstrikes. 

He also said that Joe Biden ordered further military action to disrupt and deter attacks on U.S. facilities and personnel in Iraq. Jessica McNulty, a Pentagon spokeswoman, added that these airstrikes are ongoing.

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Airstrikes in Iraq and Syria

According to Wall Street Journal (WSJ), there were no immediate reports of casualties from the action. The Pentagon noted that the U.S. has become increasingly concerned in recent months over militia groups using more sophisticated ways of attacking American forces by using remotely controlled drones.

Joe Biden did not address questions asked by reporters about the airstrikes upon his return to the White House from a weekend at Camp David.

A U.S. official said the airstrikes were done by Air Force F-15s and F-16s a little more than a week after Iran elected Ebrahim Raisi as its next president. 

The Biden administration blocked access to a series of websites linked to Iran after the nation has voted to install Raisi as its leader. Raisi is a close ally of the clerical government's supreme leader, according to The New York Times.

Iran has been using its proxy militias in Iraq to amplify pressure on the United States and other world leaders to negotiate on easing sanctions as part of a possible revival of the 2015 nuclear deal.

Meanwhile, Sen. Tim Kaine said Congress must be informed on these airstrikes without delay. Kaine is known to be a frontrunner on the fight to limit presidential war powers for a decade from the Senate Relations Committee.

The Democrat of Virginia noted that if the strikes were against militias attacking American personnel, that would be a usual self-defense action. However, he added that they need to know more.

Iran-Backed Militia Groups

The WSJ reported that Kata'ib Hezbollah is an Iranian-backed group in Iraq. The U.S. claimed that it has long organized attacks against U.S. forces.

U.S. forces had also accused the other group, Kata'ib Sayyid al-Shuhada, of carrying out the February rocket in Ebril and targeted sites linked to the group in that month's airstrike.

Michael Knights of the Washington Institute for Near East Policy said there is a strong consensus in the U.S. and Iraqi intelligence communities that Kata'ib Hezbollah is the leading operator of the drones.

Knights noted that there had been at least eight drone attacks targeting U.S. forces in Iraq from April 14 until today.

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WATCH: U.S. Conducts Airstrikes Against Iran Backed Militia Group - From NBC News