Smoke rises from ISIS positions following a U.S.-led coalition airstrike.

U.S. air strikes have been only “minimally effective” in destroying oil infrastructure in the hands of militants, a military spokesman said Wednesday, explaining why tanker trucks in Syria are now being targeted instead.

The U.S.-led coalition that has been conducting drone and plane strikes in Iraq and Syria for more than a year has repeatedly hit ISIS oil equipment – only to see the militants quickly repair it.

ISIS militants reportedly rake in millions of dollars in revenue from oil fields under their control.

“We have been striking oil infrastructure targets since the very beginning of this operation,” Colonel Steve Warren told reporters in a video call from Baghdad.

“What we found out was that many of our strikes were only minimally effective.”

On Sunday, U.S. military planners tried a new tactic, attacking a large convoy of gas trucks that had massed in the desert in eastern Syria to collect “illicit oil,” Warren said.

About 45 minutes before the strike, which saw the destruction of 116 fuel trucks, warplanes conducted a low pass known as a “show of force” and dropped leaflets warning drivers to flee.

The leaflet states: “Get out of your trucks now, and run away from them.”

“A very simple message,” Warren said.

Though the trucks were being used to support the ISIS group, the drivers were not thought to be militants, he added. The strike was conducted near Albu Kamal, an IS-held town in Deir Ezzor province along Syria’s border with Iraq.

“They’re probably just civilians,” Warren said. “So we had to figure out a way around that. We’re not in this business to kill civilians, we’re in this business to stop” the ISIS group.

War-torn Syria is also being bombed by Russia, which claims it is targeting “terrorists,” but the United States says Moscow is trying to prop up President Bashar al-Assad.

Moscow used long-range bombers that flew in from Russia on Tuesday to target several areas in Syria including the ISIS stronghold of Raqa.

Warren said it would be “no surprise” if the bombs had resulted in civilian casualties and he blasted the Russian air force as outdated.

“Those are the type of tactics needed only if you don’t possess the technology, the skills and the capabilities to conduct the type of precision strikes that our coalition conducts,” he said.

The ISIS group has declared a self-styled caliphate in bands of territory it seized in Iraq and Syria, but has faced recent setbacks from Kurdish and Arab forces in Iraq’s Sinjar and parts of northeastern Syria.

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