Jim Michaels

USA TODAY

U.S. warplanes destroyed a fleet of 168 oil tanker trucks in Syria in the largest such strike against the terror group's finances this year, the U.S.-led coalition announced Friday.

More than 20 aircraft were used to destroy the tanker trucks, which were gathered near Palmyra, in central Syria. The oil in the trucks was worth about $2 million, according to a statement.

The airstrike, which was launched Thursday, is part of an operation to reduce revenues from the Islamic State, which is also called ISIS or ISIL. The Islamic State gets 25% of its revenue from oil, the Center for the Analysis of Terrorism, a private research group, estimates.

It's not clear why the Islamic State massed so many vehicles in the open.

The first trucks that were struck did not have drivers in them and the drivers in the remaining trucks fled before they were struck, the coalition said.

Coalition aircraft have destroyed hundreds of tanker trucks and disabled infrastructure used to pump and refine oil to weaken the terror group's revenue.

The coalition has destroyed a total of 1,506 trucks as part of its campaign to target the Islamic State's oil industry.

"When ISIL has access to large sums of money, they use it to conduct violent terror attacks against anyone who doesn't share their barbaric ideology," Col. John Dorrian, a military spokesman, said in a statement.

At its peak the Islamic State, which controls territory in Iraq and Syria, had revenues of about $1.3 million a day from oil, according to the State Department.

Revenue from oil and other sources has dropped as a result of the airstrikes and the Islamic State's loss of territory in Iraq and Syria.

U.S.-backed ground forces in Iraq and Syria have won back a number of key towns and cities over the past year. Iraqi security forces are currently battling to drive militants out of Mosul, Iraq's second-largest city.

Extortion is another major revenue source for the group and the loss of territory has meant it has a smaller population to prey on.

The dramatic revenue loss is threatening the economic viability of the terror group, the Center for the Analysis of Terrorism said in a report this year.