United States fighter jets struck two sites in Iraq used by Iran-linked militants on Wednesday morning local time, according to U.S. Central Command and defense officials.

This is the fourth round of American retaliatory airstrikes in response to a near-daily spate of attacks on U.S. forces in Iraq and Syria by Iran-backed groups. The attacks began in mid-October, 10 days after Hamas' deadly terror attack on Israel. The Pentagon counts more than five dozen such attacks since then.

"U.S. forces have been attacked approximately 66 times since October 17," Pentagon Deputy Press Secretary Sabrina Singh told reporters Tuesday night. "32 separate times in Iraq, and 34 separate times in Syria."

Wednesday's strikes were partly in response to militants launching two close-range ballistic missiles at U.S. forces on al-Asad Airbase in western Iraq, injuring as many as eight service members. This was the first use of ballistic missiles against Americans in the region since mid-October, which the U.S. perceived as a definite escalation, a U.S. official told ABC News.

"On the morning of November 22 in Iraq, U.S. Central Command forces conducted discrete, precision strikes against two facilities in Iraq. The strikes were in direct response to the attacks against U.S. and Coalition forces by Iran and Iran-backed groups, including the one in Iraq on November 21, which involved use of close-range ballistic missiles," a statement from Central Command read.

The latest U.S. airstrikes in Iraq hit a command and control node, and an operations center used by Kataib Hezbollah to attack U.S. and coalition forces in Iraq and Syria recently, two U.S. officials told ABC News.

"The strikes were conducted against a Kataib Hezbollah operations center and a Kataib Hezbollah Command and Control node near Al Anbar and Jurf al Saqr, south of Baghdad," one of the officials said.

One of the officials said members of the militant group were on site during the strikes, while the other told ABC News that some were likely killed, though a full damage assessment is still under way.

In addition to the four pre-planned retaliatory airstrikes by the U.S. since Oct. 17, there have been several instances of American forces immediately firing back at Iran-linked attackers when their locations were identified.

The most recent of those counterstrikes just occurred Tuesday when an American AC-130 gunship already airborne in the region spotted a group of militants launch the aforementioned close-range ballistic missiles at U.S. troops stationed in western Iraq. The gunship crew tracked the attackers in their vehicle and struck back, killing several of them, according to U.S. officials.

"This self-defense strike resulted in some hostile fatalities," Singh said Tuesday.

The three earlier pre-planned U.S. airstrikes in recent weeks destroyed weapon storage facilities, a safe house, and another command and control center in Syria used by Iran's Islamic Revolutionary Guard Corps and its proxy militants to launch attacks, according to Pentagon officials. Some militants are expected to have been killed in at least some of those strikes.

While the U.S. counterattacks are meant to discourage further aggression, so far there has been no sign of them slowing.

"The United States does not seek conflict and has no desire to engage in further hostilities, but these Iran-backed attacks against U.S. forces are unacceptable and must stop," one of the U.S. officials said.

The official put the blame squarely on Iran.

"We continue to message to Iran that we hold them accountable for these actions, and that their leaders must take action to constrain the activities of the groups Iran directs, trains and equips. We will not hesitate to take further measures to protect our people, if necessary," the official said.