The US military struck two facilities in eastern Syria used by Iran's Islamic Revolutionary Guard Corps (IRGC) and affiliated groups, US Defense Secretary Lloyd Austin said Thursday (October 26).

"The precision self-defense strikes are a response to a series of ongoing and mostly unsuccessful attacks against US personnel in Iraq and Syria by Iranian-backed militia groups that began on October 17," he said in a statement.

One US citizen contractor died from a cardiac incident during the attacks, and 21 US military personnel suffered "minor injuries, but all have since returned to duty," Austin added.

But the Pentagon announced late Thursday that 19 of the US military members had suffered traumatic brain injuries after the attacks.

"These narrowly tailored strikes in self-defense were intended solely to protect and defend US personnel in Iraq and Syria," Austin said. "They are separate and distinct from the ongoing conflict between Israel and Hamas, and do not constitute a shift in our approach to the Israel-Hamas conflict."

"The United States does not seek conflict and has no intention nor desire to engage in further hostilities, but these Iranian-backed attacks against US forces are unacceptable and must stop," he said.

The strikes in Syria were conducted by two Air Force F-16 jets, accompanied by MQ-9 drones, against "a weapons storage facility and an ammunition storage facility near Albu Kamal," the New York Times (NYT) reported.

"While rockets or drones had not been launched from the site, officials said munitions stored at the facilities were used in the recent attacks against US forces," the newspaper said.

The strikes were intended to send a strong signal to Iran to rein in its attacks against US troops in Syria and Iraq "without escalating the conflict in the Middle East," US officials said.

A senior US Defense Department official told reporters late Thursday that the airstrikes were not coordinated with Israel's military, according to the NYT.

Warning Iran against escalation

The precision strikes follow a direct warning earlier Thursday from US President Joe Biden to Iran's leader Ali Khamenei against attacks on US troops.

"There was a direct message relayed. That's as far as I'm going to go," US National Security Council spokesman John Kirby told reporters, declining to say how it was delivered.

Biden told a press conference on Wednesday that he had warned Khamenei of a response if the attacks continue.

"My warning to the Ayatollah was that if they continue to move against those troops, we will respond, and he should be prepared. It has nothing to do with Israel," he said.

The US Defense Department says that US and allied forces in Iraq and Syria have been attacked by "Iranian-backed militia groups" at least 16 times this month as tensions rise in the region.

The US military has deployed a vast array of firepower into the Middle East as part of ongoing efforts to help defend its ally Israel and to deter malign actors in the region from further escalating the conflict.

The deployments include two aircraft carriers -- the USS Ford Carrier Strike Group (CSG) and the USS Eisenhower -- and the 26th Marine Expeditionary Unit (MEU).

The 26th MEU is designated as "special operations-capable" (SOC), which means it is capable of specialized tasks and working alongside other special operations forces.

Designed to be versatile and quick to respond, an MEU usually has 2,200 Marines and includes a battalion landing team, composite helicopter squadron and combat logistic battalion.