The United States has carried out a new round of airstrikes against Iran-backed proxy militia in Iraq and Syria, a statement released by the military said on Sunday Washington time.

The strikes were in response to recent drone attacks by those militia against US forces and facilities in Iraq. The Iranian trained and supplied Shiite groups have started using explosive drones, instead of Katyusha rockets or mortars used in the past.

The US military said it targeted operational and weapons storage facilities at two locations in Syria and one location in Iraq. It did not disclose whether it believed anyone was killed or injured but officials said assessments were ongoing.

The strikes were ordered by President Joe Biden as the militia continued their attacks on US targets amid nuclear negotiations with Iran in Vienna. This was the second time since February that Biden ordered a strike against these militias.

Pentagon Press Secretary John Kirby said the militias were using the facilities to launch unmanned aerial vehicle attacks against US troops in Iraq. He described the airstrikes as “defensive,” saying they were launched in response to the attacks by militias.

“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,” Kirby said.

Two Iraqi militia officials told The Associated Press in Baghdad that four militiamen were killed in the airstrikes near the border with Syria. Both officials spoke on condition of anonymity because they were not authorized to give official statements.

The Syrian Observatory for Human Rights, a Britain-based group that closely monitors the Syrian conflict through activists on the ground, reported that at least five Iraqi militiamen were killed in the airstrikes.

Biden’s decision to authorize the strike becomes more significant as his administration has been negotiating with Tehran since early April to restore the 2015 nuclear agreement for President Trump abandoned in 2018. Diplomats say that the talks have reached a critical stage when political decisions should be made by both sides.

Kirby said Biden "has been clear that he will act to protect US personnel. Given the ongoing series of attacks by Iran-backed groups targeting U.S. interests in Iraq, the President directed further military action to disrupt and deter such attacks."

The Pentagon spokesman added: “As a matter of international law, the United States acted pursuant to its right of self-defense. The strikes were both necessary to address the threat and appropriately limited in scope.”

US officials believe Iran is behind a ramp-up in increasingly sophisticated drone attacks and periodic rocket fire against U.S. personnel and facilities in Iraq.

Two US officials, speaking to Reuters on condition of anonymity, said Iran-backed militias carried out at least five drone attacks against facilities used by U.S. and coalition personnel in Iraq since April.

An escalation in attacks by Iranian proxies in Iraq in December 2019 culminated in the targeted killing of Qasem Soleimani Iran’s top operator in the Middle East, ordered by Trump.

House Speaker Nancy Pelosi said in a statement Sunday that the US airstrikes “appear to be a targeted and proportional response to a serious and specific threat,” adding, “Protecting the military heroes who defend our freedoms is a sacred priority."

With reporting by Reuters and Associated Press