U.S. Defense Secretary Lloyd Austin on Monday vowed that the United States would take "all necessary actions" to defend its troops after a drone attack by Iran-backed militants that killed three U.S. service members and wounded dozens more, Paralel.Az reports citing Reuters.

The attack on Sunday was the first deadly strike against U.S. troops since the Israel-Hamas war erupted in October, and marks a major escalation in tensions that have engulfed the Middle East.

"Let me start with my outrage and sorrow (for) the deaths of three brave U.S. troops in Jordan and for the other troops who were wounded," Austin said at the Pentagon.

"The president and I will not tolerate attacks on U.S. forces and we will take all necessary actions to defend the U.S. and our troops," Austin added at the start of meeting with NATO Secretary General Jens Stoltenberg at the Pentagon.

The United States is trying to determine exactly why the nearly 350 troops at the base in Jordan, known as Tower 22, were unable to stop the drone.

Two officials said a U.S. drone was approaching the base around the same time the attack drone was incoming. One of the officials said the attack drone was also flying low, factors that may have contributed to it being missed by base defenses.

U.S. troops have been attacked over 150 times in Iraq, Syria and Jordan, as well as on warships in the Red Sea, where Houthi fighters in Yemen have been firing drones and missiles at them.

The attacks are piling political pressure on President Joe Biden to deal a blow directly against Iran, a step he has been reluctant to take out of fear of igniting a broader war.

Biden's options could range anywhere from targeting Iranian forces outside to even inside Iran, or opting for a more cautious retaliatory attack solely against the Iran-backed militants responsible, experts say.

"Iran continues to destabilize the region, this includes backing terrorists who attack our ships in the Red Sea," Stoltenberg said.

At least 34 personnel were injured in the Jordan attack, but that number is expected to change as more people seek care. Eight personnel were evacuated from Jordan for higher-level care, but are in stable condition.