The U.S. carried out airstrikes in Iraq and Syria.

The United States carried out airstrikes early Monday morning in Iraq and Syria against two Iranian-backed militias that the Pentagon said had conducted drone strikes against American personnel in Iraq in recent weeks, the Defense Department said.

“At President Biden’s direction, U.S. military forces earlier this evening conducted defensive precision airstrikes against facilities used by Iran-backed militia groups in the Iraq-Syria border region,” the Pentagon spokesman, John F. Kirby, said in a statement.

Mr. Kirby said the facilities were used by Iranian-backed militias, including Kata’ib Hezbollah and Kata’ib Sayyid al-Shuhada, to store arms and ammunition for carrying out attacks against places where Americans were located in Iraq. There were no immediate reports of casualties but a military after-action review is ongoing, Pentagon officials said.

The strikes were the second time that Mr. Biden has ordered the use of force in the region. The United States carried out airstrikes in eastern Syria in late February against buildings belonging to what the Pentagon said were Iran-backed militias responsible for attacks against American and allied personnel in Iraq.

Pressure has been building for weeks from Democrats and Republican in Congress, and from some of Mr. Biden’s top advisers and commanders, to retaliate against the threat posed by the drones to American diplomats and the 2,500 U.S. troops in Iraq who are training and advising Iraqi forces.

At least five times since April, the Iranian-backed militias have used small, explosive-laden drones that divebomb and crash into their targets in late-night attacks on Iraqi bases — including those used by the C.I.A. and U.S. Special Operations units, according to American officials. So far, no Americans have been hurt in the attacks, but officials worry about the precision of the drones, also called unmanned aerial vehicles, or U.A.V.’s.

Iran — weakened by years of harsh economic sanctions — is using its proxy militias in Iraq to step up pressure on the United States and other world powers to negotiate an easing of those sanctions as part of a possible revival of the 2015 nuclear deal.