WASHINGTON — The US military carried out airstrikes at sites in Syria and Iraq on Friday night in retaliation for a drone attack that killed three US soldiers at a base in Jordan last Sunday, two US defense officials confirmed to Al-Monitor.

The strikes are believed to be directed at dozens of targets linked to militias backed by Iran's Islamic Revolutionary Guard Corps (IRGC).

Following a series of meetings with top national security advisors, President Joe Biden on Wednesday said he had decided on the response, which other administration officials said would come in phases.

White House National Security Council coordinator John Kirby said the first action taken in response to the Tower 22 attack wouldn't "be the last" seen from the US.

"We have called on the Iranian proxy groups to stop their attacks. They have not, and so we will respond in a time and manner of our choosing,” Pentagon Press Secretary US Air Force Maj. Gen. Patrick Ryder vowed on Tuesday.

While no specific militia claimed credit for the attack, the Pentagon has said it bore “the footprints” of Kataib Hezbollah. White House and Pentagon officials this week attributed the lethal incident to the self-styled "Islamic Resistance" movement in Iraq, an umbrella term for Iran-backed militias that includes Kataib Hezbollah and others.

In an apparent last-ditch attempt to save face, Kataib Hezbollah announced on Tuesday it was suspending attacks against US forces in Iraq and Syria, implying Iran was not privy to its attacks. The statement emerged amid a visit by the commander of the Quds Force of Iran's IRGC to Baghdad.

"The president and I will not tolerate attacks attacks on American troops, and neither will I," US Defense Secretary Lloyd Austin said Thursday in his first press briefing at the Pentagon in more than a year.

Asked by Al-Monitor why the US had decided to respond in phases, rather than a single decisive retaliation, Austin replied, "I don't think the adversaries are of a ‘one-and-done’ mindset. They have a lot of capability. I have a lot more."

This is a breaking story and will be updated.