The United States started carrying out retaliatory strikes on Friday in Iraq and Syria, three U.S. officials said, after a deadly drone attack by Iran-backed militants in Jordan that killed three U.S. troops and injured some 40 others.

U.S. forces struck Iranian proxy forces in half a dozen sites, a U.S. military official told the New York Times. The attacks, which reportedly have not targeted sites within Iran, are likely to be the start of a larger-scale operation against Iran-backed forces in the region, according to CNN.

The U.S. strikes came hours after President Joe Biden attended a military ceremony honoring the U.S. troops killed in the Sunday drone attack as their remains were returned to the United States. Biden has faced pressure, particularly from Republicans, to retaliate decisively against Iran in the wake of the soldiers' deaths.

U.S. officials have assessed that Iran manufactured the drone used in the attack, though they have also made comments downplaying Iran's responsibility for the militants' attack, telling media outlets that Iran has limited control over its proxy groups.

Amid early reports of the United States' retaliatory strikes on Friday, Sen. Lindsey Graham (R., S.C.) described the response as insufficient.

"The lapse of time has lessened the impact of deterrence," Graham told Fox News. "And by not hitting oil infrastructure in Iran or Revolutionary Guard personnel, you will have failed to make the point."