COLUMBIA, S.C. (AP) — Three American service members were killed and “many” were wounded in a drone strike in northeast Jordan near the Syrian border, President Joe Biden said in a statement Sunday. He attributed the attack to Iran-backed militia groups.

They were the first U.S. fatalities in months of strikes against American forces across the Middle East by Iranian-backed militias amid the Israel-Hamas war in Gaza, increasing the risk of escalation. U.S. officials were still working to conclusively identify the precise group responsible for the attack, but have assessed that one of several Iranian-backed groups is to blame.

Biden said the United States “will hold all those responsible to account at a time and in a manner (of) our choosing.”

Jordanian state television quoted Muhannad Mubaidin, a government spokesman, as insisting the attack happened outside of the kingdom across the border in Syria. U.S. officials insisted that the attack took place in Jordan.

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 said 25 service members were injured the attack in addition to the three killed.

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

Since Israel’s war on Hamas in the Gaza Strip began, U.S. troops in Iraq and Syria have faced drone and missile attacks on their bases. The attack on Jordan marks the first targeting American troops in Jordan during the war and the first to result in the loss of American lives. Other attacks have left troops seriously injured, including with traumatic brain injuries.

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 Iranian-backed Houthi rebels from continuing to threaten commercial shipping in the Red Sea.

Biden, who was in Columbia, South Carolina, on Sunday, was briefed 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. He was expected to meet again with his national security team later Sunday.

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.

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Associated Press writers Bassem Mroue in Beirut, Omar Akour in Amman, Jordan and Jon Gambrell in Jerusalem contributed to this report.