Pentagon building aerial FILE
CNN  — 

Military bases in Iraq and Syria that hold American troops were attacked Wednesday, though no US forces were killed in the strikes, according to Iraqi officials and the US-led coalition.

There have been multiple attacks this week coinciding with Monday’s second anniversary of the US assassination of a key Iranian general.

“It’s difficult to know with great specificity and certainty … what accounts for the frequency of these attacks,” Pentagon press secretary John Kirby told reporters. “It is certainly possible that it could be related to the anniversary of the Soleimani strike. It is certainly possible that it could be related to the change in mission” in Iraq.

“The truth is, we just don’t know for sure,” he added. “Bottom line is we had been thinking and preparing for the possibility of stepped-up attacks at the end of December.”

On Wednesday morning, US-led coalition forces fired back after they were attacked by suspected Iranian-backed militias who fired eight rounds of indirect fire at their base in eastern Syria near the Iraqi border, according to a statement from the coalition.

While the attack at the Green Village base did not result in any deaths, several rounds landed on the base and caused minor damage, the statement said.

The coalition “responded swiftly and fired six rounds of artillery towards the point of origin of the attack just outside Mayadin, Syria,” according to the statement.

In a separate attack on Wednesday, at least five rockets landed inside the Iraqi military’s al-Asad base in western Anbar province without causing any casualties, Sharhabil al-Obaidi, mayor of Al Baghdadi town, and Suleiman al-Kubaisi, media adviser to the governor of Anbar, told CNN. The Al-Asad base also hosts US forces.

Sources said the rockets were fired from al-Bastamiya village, 15 kilometers – about 9 miles – east of al-Asad base.

Following the incident, the Iraqi military said in a statement that rockets had been fired by “outlaw elements” and security forces are conducting search and investigation operations.

Earlier Wednesday, a rocket had landed inside the Camp Victory base near Baghdad International Airport with no reports of material or human losses, the Iraqi military said.

Wednesday’s attacks come a day after the US military conducted strikes in Syria after indirect fire posed what a US-led coalition official called “an imminent threat” to troops near Green Village.

Earlier on Tuesday, the US shot down two drones as they approached Ain al-Asad Air Base in Iraq, according to the Iraqi Prime Minister’s Office. It is the same base targeted by Iranian missiles two years ago after the US assassination of Islamic Revolutionary Guard Corps Gen. Qasem Soleiman

Tensions are also high as sensitive negotiations continue in Vienna, Austria, on the future of the Iran nuclear deal.

The US maintains approximately 900 troops in Syria, largely split between At-Tanf and Green Village.

This story has been updated with additional details Wednesday.