Three American troops were killed and dozens were injured Sunday in a drone strike in northeast Jordan near the Syrian border, the U.S. military said. President Joe Biden blamed Iran-backed militias for the first U.S. fatalities after months of strikes by the groups against American forces across the Middle East amid the Israel-Hamas war. 

With an increasing the risk of military escalation in the region, U.S. officials were working to conclusively identify the precise group responsible for the attack, but they have assessed that one of several Iranian-backed groups was behind it. 

Biden said the United States “will hold all those responsible to account at a time and in a manner (of) our choosing.” Defense Secretary Lloyd Austin said “we will take all necessary actions to defend the United States, our troops, and our interests.” 

Iran-backed fighters in east Syria began evacuating their posts, fearing U.S. airstrikes, according to Omar Abu Layla, a Europe-based activist who heads the Deir Ezzor 24 media outlet. He told The Associated Press that the areas are the strongholds of Mayadeen and Boukamal. 

According to a U.S. official, the number of troops injured in the attack by the one-way attack drone rose to at least 34. Another official, who also spoke on condition of anonymity to discuss details not made public, said a large drone struck the base, which two other American officials identified as an installation in Jordan known as Tower 22. It is along the Syrian border and is used largely by troops involved in the advise-and-assist mission for Jordanian forces. 

The small installation, which Jordan does not publicly disclose, includes U.S. engineering, aviation, logistics and security troops. Defense Secretary Lloyd Austin said the troops were deployed there “to work for the lasting defeat of ISIS.” Three officials said the drone struck near the troops’ sleeping quarters, which they said explained the high casualty count. 

The U.S. military base at al-Tanf in Syria is just 20 kilometers (12 miles) north of Tower 22. The Jordanian installation provides a critical logistical hub for U.S. forces in Syria, including those at al-Tanf, which is near the intersection of the Iraq, Syria and Jordan borders. 

Jordanian state television quoted Muhannad Mubaidin, a government spokesman, as insisting the attack happened across the border in Syria. 

U.S. troops long have used Jordan, a kingdom bordering Iraq, Israel, the Palestinian territory of the West Bank, Saudi Arabia and Syria, as a basing point. U.S. Central Command put the toll at three killed and 25 injured. 

Some 3,000 American troops typically are stationed in Jordan. 

Since the war in Gaza began Oct. 7, Iranian-backed militias have struck American military installations in Iraq more than 60 times and in Syria more than 90 times, with a mix of drones, rockets, mortars and ballistic missiles. The attack Sunday was the first targeting American troops in Jordan during the Israel-Hamas war and the first to result in the loss of American lives. Scores of U.S. personnel have been wounded, including some with traumatic brain injuries, during the attacks. 

The militias have said that their strikes are in retaliation for Washington’s support for Israeli in the war in Gaza and have also said they aim to push U.S. forces out of the region. 

The U.S. in recent months has struck targets in Iraq, Syria and Yemen to respond to attacks on American forces in the region and to deter Iran-backed Houthi rebels from continuing to threaten commercial shipping in the Red Sea. 

“I am confident the Biden Administration will respond in a deliberate and proportional manner,” said said Sen. Jack Reed, D-R.I., who heads the Senate Armed Services Committee. 

Republicans in Congress said the administration’s approach had failed to deter America’s adversaries in the region. 

“We need a major reset of our Middle East policy to protect our national security interests,” said Republican Rep. Michael McCaul of Texas, chairman of the House Foreign Affairs Committee. Sen. Lindsay Graham, R-S.C., went further, urging the administration “to strike targets of significance inside Iran, not only as reprisal for the killing of our forces, but as deterrence against future aggression. The only thing the Iranian regime understands is force.” 

Biden, who was in Columbia, South Carolina, on Sunday, was briefed in the morning by Defense Secretary Lloyd Austin, national security adviser Jake Sullivan, and principal deputy national security adviser Jon Finer, White House press secretary Karine Jean-Pierre said. In the afternoon, he met virtually with Vice President Kamala Harris and his national security team for an update. 

The president called it a “despicable and wholly unjust attack” and said the service members were “risking their own safety for the safety of their fellow Americans, and our allies and partners with whom we stand in the fight against terrorism. It is a fight we will not cease.” 

Syria is still in the midst of a civil war and long has been a launch pad for Iranian-backed forces there, including the Lebanese militia Hezbollah. Iraq has multiple Iranian-backed Shiite militias operating there as well. 

Jordan, a staunch Western ally and a crucial power in Jerusalem for its oversight of holy sites there, is suspected of launching airstrikes in Syria to disrupt drug smugglers, including one that killed nine people earlier this month. 

An umbrella group for Iran-backed factions known as the Islamic Resistance in Iraq earlier claimed launching explosive drone attacks targeting three areas in Syria, as well as one inside of “occupied Palestine.” The group has claimed responsibility for dozens of attacks against bases housing U.S. troops in Iraq and Syria since the Israel-Hamas war began. 

Three officials with Iran-backed militias in Iraq, who spoke on condition of anonymity because they were not authorized to discuss the matter with journalists, said the drone attack against the base in Jordan was launched by one of the Iraqi groups. No faction has yet officially claimed responsibility. 

Officials said the U.S. military is not tracking any other attacks on its forces Sunday in the region.