Saturday's strikes in Yemen followed ones in Iraq and Syria the day prior.
The United States on Friday began to carry out airstrikes against Iran-backed militants and Iranian military targets in Iraq and Syria in retaliation for a drone attack on an American base in Jordan on Jan. 28 that killed three U.S. service members.
Dozens of other American troops were wounded in the drone attack on the Tower 22 base near Jordan's border with Iraq and Syria. The U.S. says Iran is responsible for funding and arming the militants while Iran has denied involvement.
U.S. President Joe Biden had quickly warned that America would respond forcefully, escalating U.S. involvement in the Middle East after months of trying to contain tensions from boiling over into a broader war in the region.
The U.S. has attributed the drone attack on the American base in Jordan to the Islamic Resistance in Iraq, an umbrella group of Iran-backed militias.
According to a U.S. official, the drone that successfully hit the base was an Iranian-made Shahed drone, similar to those used by the Russians on the battlefield in Ukraine.
Iran has denied involvement in the Jordan strike. But President Joe Biden earlier this week faulted Iran for providing munitions and funding to these different proxy groups.
The U.S. base is known as Tower 22, a major logistical hub for U.S. troops still in Syria on a mission to prevent a resurgence by Islamic State fighters. According to Central Command, there are approximately 350 U.S. Army and Air Force personnel deployed to the base.
The Syrian Observatory for Human Rights, a group which reports on war in Syria, confirmed that 29 members of Iranian militias were killed in airstrikes on 28 positions.
Separately, Iraq's Popular Mobilization Forces, a state security force including Iran-backed groups, said that 16 its members were killed including fighters and medics.
In a statement on Saturday, the Hezbollah terrorist organization strongly condemned the U.S.'s strikes on Iraq and Syria and extended its sympathies for the lives lost.
"What the United States of America did is a blatant violation of the sovereignty of the two countries, an attack on their security and territorial integrity, and a shameless violation of all international and humanitarian laws," Hezbollah said.
"This new aggression contributes to destabilizing the region, and creating false justifications and pretexts for the continuation of the American occupation of several regions in Iraq and Syria against the will of their people who yearn for freedom and independence," Hezbollah said.
U.S. officials said Friday that targets were chosen to avoid civilian casualties and because they were connected to enabling the attacks against American service members.
-ABC News' Ghazi Balkiz and Aras Maman
A Jordanian military source told the state-owned Al-Mamlaka TV that Jordan was not involved in U.S. strikes on Iraqi soil, refuting earlier allegations. However, the Jordanian statement did not mention strikes in Syrian territory.
“There is no truth to press reports regarding the participation of Jordanian aircraft in operations carried out by American aircraft inside Iraq," the source told Al-Mamlaka TV.
"The Jordanian Armed Forces respect the sovereignty of brotherly Iraq," the source said, confirming "the depth of the brotherly relations that unite Jordan with all Arab countries."
-ABC News' Ghazi Balkiz and Aras Maman
The Iraqi government declared that it will have three days of public mourning over the civilians and armed forces who were killed in the U.S. strikes.
"Today, Saturday, the Prime Minister, Mr. Muhammad Shiaa Al-Sudani, directed the declaration of general mourning in all state departments and institutions, for a period of three days, out of mercy for the souls of the martyrs of our armed forces and the civilians who died as a result of the American bombing on the areas of Akashat and Al-Qaim, west of Anbar Governorate," the Iraqi prime minister's office said in a statement.
-ABC News' Edward Szekeres
