U.S. Secretary of Defense Lloyd Austin speaks during a press conference at the Pentagon last week.

U.S. Secretary of Defense Lloyd Austin speaks during a press conference at the Pentagon last week. / Getty Images

Updated March 24, 2023 at 12:58 PM ET

U.S. forces in eastern Syria suffered a second drone attack Friday morning as part of tit-for-tat of strikes between the U.S. and militia groups supported by Iran, the Pentagon said.

Officials confirmed to NPR that the latest strike did not cause damages or injuries to the U.S. forces or their partners.

The latest incident came just a day after a militia drone strike killed a U.S. contractor, injured five American service members and wounded one other contractor in Syria, the Defense Department said on Thursday.

Intelligence sources say the drone, which struck a U.S. base near Hasakah, was made by Iran, Defense Secretary Lloyd Austin said in a statement.

In response, the U.S. Central Command conducted "precision airstrikes" on groups it says were connected with Iran's Revolutionary Guard Corps.

"As President Biden has made clear, we will take all necessary measures to defend our people and will always respond at a time and place of our choosing," Austin said. "No group will strike our troops with impunity."

Videos shared on social media showed explosions in Deir el-Zour, a Syrian province near the Iraq border that contains oil fields.

That area is controlled by Iran-backed militia groups and Syrian forces, according to the Associated Press, and has seen recent damage from suspected airstrikes by Israel, targeting alleged Iranian supply routes.

The American strikes on Thursday killed 11 people, according to the Syrian Observatory for Human Rights, a group that monitors violence in the region with a network of local sources. Six were Iranian-backed militants in a warehouse in the city. Two were killed on the outskirts of a surrounding town, Al-Mayadeen. Three others were killed in the surrounding desert.

"The number of fatalities is expected to rise, as the strikes left several militiamen injured, some seriously," the group wrote online.

Neither Iran nor Syria appeared to immediately acknowledge the strikes.

The attack and retaliation pose threats to recent negotiations aimed at de-escalating tensions across the Middle East. Earlier this month, Iran and Saudi Arabia agreed to restore diplomatic ties in a deal brokered by China, raising questions about whether U.S. influence in the region is at risk of being overshadowed.

The U.S. has had ground troops in northeastern Syria since 2015 when it was fighting the Islamic State.

Syria's civil war, which erupted in 2011, has morphed into a wider proxy conflict that includes U.S. forces supporting Kurdish fighters, and Iran supporting militants.

Austin said that Thursday's retaliatory airstrikes were not just a response to the most recent drone strike, but also to a "series of recent attacks against coalition forces in Syria."

U.S. soldiers participate in a joint military exercise in northeastern Hasakah province in September 2022.

U.S. soldiers participate in a joint military exercise in northeastern Hasakah province in September 2022. / AFP via Getty Images

Iran's Revolutionary Guard has launched an estimated 78 attacks on U.S. forces in Syria since January 2021, U.S. Army General Michael "Erik" Kurilla, who heads Central Command, told the U.S. House Armed Services Committee on Thursday.

The U.S. Central Command, which was formed in 1983 to monitor Iranian expansion, has raised alarm about the expansion of the Iran's military strength.

The country's Islamic Revolutionary Guard Corps, a paramilitary group that reports to Supreme Leader Ali Khamenei, relies on proxy militia groups throughout the region.

In written comments to the U.S. House committee, Kurilla said that deterring Iran is "arguably more urgent than at any time in CENTCOM's history" because the IRGC is able to arm those proxy groups with increasingly sophisticated weapons like uncrewed drones, as well as ballistic and cruise missiles.

Iran has sold advanced drones to other nations, including Russia for use against Ukraine.

"Iran possesses the largest and most diverse missile arsenal in the Middle East," Kurilla said. "Even more concerning, Iran has advanced its nuclear program such that Tehran can now produce sufficient fissile material for a nuclear weapon in less than 14 days."

Talks about reviving the 2015 Iran Nuclear Agreement, which former President Donald Trump abandoned in 2018, continue to stall.

In a statement following Thursday's airstrikes, Kurilla said the Army was "postured for scalable options in the face of any additional Iranian attacks."

"We will always take all necessary measures to defend our people," he wrote.

Two of the service members were treated on site, the Pentagon said. Three others were medically evacuated to medical facilities in Iraq.

This is a developing story. Check back for updates.

NPR's Peter Kenyon and Andrew Sussman contributing reporting.

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