WASHINGTON (AP) -- In a new twist to an intensifying campaign to squeeze Islamic State oil revenues, U.S. warplanes have destroyed 116 oil-hauling trucks in eastern Syria that were a key part of a smuggling operation that brings the group an estimated $1.4 million a day.
The series of strikes conducted Sunday and announced Monday was the first of their kind in more than a year of U.S.-led airstrikes in Syria. Four A-10 Thunderbolt II attack planes and two AC-130 Spectre gunships pounded the trucks as they clustered near Abu Kamal, a town close to the Iraqi border.
U.S. officials previously had said they avoided attacking fuel trucks out of concern for civilian casualties.
Navy Capt. Jeff Davis, a Pentagon spokesman, said that in an effort to warn the truck drivers to leave the area in advance of Sunday's attacks, leaflets were dropped and coalition planes conducted low-level "show of force" flights over the site.
Davis said the coalition had determined that more needed to be done to inhibit the Islamic State's generation of oil revenues in Syria and Iraq. The Treasury Department said last year that the group earned nearly $1 million a day from illicit oil sales; the Pentagon believes that sum has risen to nearly $1.4 million a day. Since the earliest days of the U.S.-led bombing campaign, some parts of the Islamic State's oil infrastructure have been attacked, but the effort is now intensifying.
"This part of it was designed to attack the distribution component of ISIL's oil smuggling operation," Davis said, referring to attacking the fuel trucks. "ISIL is stealing oil from the people of Iraq and Syria to fund its campaign of terror."
Although Sunday's strikes came just two days after the Paris attacks for which the Islamic State had claimed responsibility, Pentagon officials said there was no direct connection between the two events.
The attacks were part of a broader U.S.-led coalition campaign to cripple a key source of revenue for the Islamic State. Davis said it would take "some time" to fully realize the long-term effects of targeting key elements of the oil network.
"In the short term we know we are disrupting a significant source of funding that's being used to kill innocent people," he said.
Asked why the U.S. had waited so long to hit fuel trucks, which are highly vulnerable and easily targeted by air, Davis said the U.S.-led coalition has been mindful of civilian casualties and sought to limit damage to Syria's oil resources to preserve them for future generations.
"We're balancing that with the fact that this revenue is presenting a clear and present threat to Syrians today, in that it's being converted into funds which are being used for military equipment, which is being used to kill innocent civilians," Davis said.
Army Col. Steve Warren, a U.S. military spokesman in Baghdad, said last week that the U.S. has learned over time that the militants were quickly replacing or repairing damage to oil distribution centers and other infrastructure. In many cases, he said, within days of an airstrike the targeted piece of oil infrastructure was up and running again.
"We don't want to completely and utterly destroy these facilities to where they're irreparable," Warren said. "So what we've done is we've used very precise targeting, a very detailed analysis to strike certain parts of these facilities that will cause them to shut down for an extended period of time."
Warren said the military did detailed analysis to determine how to knock out the facilities for more than a day or two.
"We wanted them broken longer. Rather than 24 to 48 hours, we're looking at, you know, something that would take maybe a year to repair," he said.
The goal, he said, was to destroy machinery or facilities for which the militants don't have the needed replacement parts.
The overall campaign against oil infrastructure, dubbed Operation Tidal Wave II, is about 70 percent complete, Warren said. He said Army Lt. Gen. Sean MacFarland, the top U.S. commander in charge of the counter-Islamic State campaign, approved the operation, which was named after a World War II mission - Operation Tidal Wave, conducted against Nazi oil fields in Romania in August 1943.