For the second time in nearly two weeks, the United States carried out airstrikes against a facility used by Iran’s Islamic Revolutionary Guards Corps and its proxies in eastern Syria early Thursday, ratcheting up retaliation for a steady stream of rocket and drone attacks against American forces in Iraq and Syria.

The strikes by two Air Force F-15E jets against a weapons warehouse in Deir al-Zour Province, Syria, came after U.S. airstrikes on Oct. 27 against similar targets in eastern Syria failed to deter Iran or its proxies in Syria and Iraq, which the Biden administration has blamed for the attacks.

Not only have the attacks continued—there have been at least 22 more since the American retaliatory strikes last month—but Pentagon officials said they have become more dangerous. Iran-backed militias have packed even larger loads of explosives—more than 80 pounds—onto drones launched at American bases, U.S. officials said.

“This precision self-defence strike is a response to a series of attacks against U.S. personnel in Iraq and Syria by I.R.G.C.-Quds Force affiliates,” Defence Secretary Lloyd J. Austin III said in a statement. “The president has no higher priority than the safety of U.S. personnel, and he directed today’s action to make clear that the United States will defend itself, its personnel, and its interests.”

“The United States is fully prepared to take further necessary measures to protect our people and our facilities,” he added. “We urge against any escalation.”

The downing of the drone, the mainstay of the American military’s aerial surveillance fleet, was another escalation of violence between the United States and Iran-backed groups in the region. The episode underscored the risks that the conflict between Israel and the Palestinian group Hamas could spiral into a wider war.

Biden administration officials have been trying to calculate how to deter the Iranian-backed Shiite militias from attacking American troops in the region without sparking that broader conflict, said three administration officials, who spoke on the condition of anonymity to discuss internal planning.

The Pentagon said on Wednesday that there had been at least 41 attacks on U.S. forces in Syria and Iraq since Oct. 17 and that at least 46 U.S. service members had been injured, 25 of whom had suffered traumatic brain injuries. The United States has 2,500 troops in Iraq and 900 in Syria, mostly to help local forces fight remnants of the Islamic State.