The United States and Russia are going after the Islamic State group's oil industry, destroying refineries and hundreds of tanker trucks transporting oil from eastern Syria in a heavy bombardment in recent days aiming to break the extremists' biggest source of income.

The campaign already appears to be having some effect, with oil prices rising in areas of Syria that rely on crude smuggled out of IS areas. But experts say it will be difficult to cut off the militants' trade completely since they are likely to switch to smaller, more elusive vehicles.

Putting a total end to the industry would mean destroying the oil fields in Syria, but that would also bring hardship to millions in the population under IS rule and others who depend on the group's oil, causing fuel shortages as winter sets in. Otherwise, taking the fields would require ground forces.

Still, the campaign could hit hard on an industry that U.S. officials say generates more than half the revenue the Islamic State group uses to maintain its rule over its swath of territory across Syria and Iraq and pay its fighters.

IS controls almost all of Syria's oil fields, concentrated in the east of the country, producing some 30,000 barrels a day, along with one field in Iraq. It smuggles most abroad, mainly to Turkey, selling at cut-rate prices and generating nearly $50 million a month.

In the wake of the bloody attacks in Paris last week — and the downing of a Russian passenger jet in Egypt's Sinai widely blamed on IS — American and Russian warplanes unleashed a stepped-up wave of strikes on oil infrastructure.

Russia's bombing blitz this week by warplanes and cruise missiles from navy ships destroyed 15 oil refining and storage facilities in Syria and 525 trucks carrying oil, Defense Minister Sergei Shoigu said Friday. Aerial footage released by the military showed airstrikes hitting a column of oil tankers in the Syrian desert, and sections of a large oil refinery bursting into flames.

On Sunday, U.S. defense officials said warplanes destroyed 116 oil-hauling trucks in eastern Syria, the biggest strikes on the oil trade since the U.S.-led air campaign began more than a year and a half ago. Attack planes and gunships pounded the trucks as they clustered near an oil facility in Abu Kamal, a town close to the Iraqi border. On Nov. 9, a French airstrike hit an oil supply center in the eastern Syrian province of Deir el-Zour.

The U.S.-led coalition has targeted oil infrastructure occasionally in the past, including a heavy attack last month on Syria's Omar field near the town of Deir el-Zour that hit refineries, command and control centers and transportation nodes.

"Degrading this source of revenue will reduce ISIL's ability to fund their military and terrorist operations," said Col. Steven Warren, the spokesman for the U.S.-led coalition in Baghdad, using another acronym for the group.

"Oil is as important to ISIS as weapons because it needs this to fund its operations. In some cases I call it the driver of military operations for ISIS," said Ahmed Ali, an Iraq analyst and senior fellow at the Institute of Regional and International Studies at the American University of Iraq.

It is not clear why coalition forces waited this long before going after IS oil infrastructure. U.S. officials previously had said they avoided attacking fuel trucks out of concern for civilian casualties. In Sunday's strikes, warplanes first dropped leaflets telling drivers to flee, reading, "Airstrikes are coming. Oil trucks will be destroyed. Get away from your oil trucks immediately. Do not risk your life."

Combined with recent gains by Kurdish forces and their Arab allies that have cut off some of the main supply routes between IS strongholds in Iraq and Syria, the airstrikes are likely to deal a painful blow to the group.

Last week, Kurdish forces captured the town of Sinjar in northwestern Iraq, severing of a highway serving as a supply route for Islamic State fighters between the northern Iraqi city of Mosul and the militant's self-proclaimed capital of Raqqa in Syria. An Iraqi oil refinery also has been taken from the militants, and Iraqi forces have encircled the western Iraqi city of Ramadi.

It is too soon to tell to the impact of this week's airstrikes on IS operations. But there are signs the campaign is already causing prices to rise.

Rami Abdurrahman, director of the Syrian Observatory for Human rights, said prices of fuel shot up in some IS-ruled areas by around 80 percent, due to truck drivers refusing to drive to IS oil facilities, fearing their vehicles will be hit.

Oil prices have increased and availability decreased in and around the IS-held Iraqi city of Mosul, said Ben Lando, editor in chief of the Iraq Oil Report, a trade publication that tracks the Iraqi oil industry. He said that is an indicator that airstrikes, as well as the cutting of supply routes, have made a difference.

Many parts of Syria controlled by other rebels — even ones fighting the Islamic State group — rely on IS for oil since they have few other sources. Even the Syrian government is said by Western diplomats and Syrian opposition member to buy oil from IS through middlemen, though Syrian officials deny it — and the main source of oil for government-held parts of the country is shipments from Damascus' top ally Iran.

A local rebel commander in Idlib said there is an indirect deal between IS and various rebel factions, under which IS supplies them with oil in return for fruits and vegetables, since IS-ruled regions largely rely on imports of produce. The commander spoke on condition of anonymity to discuss the sensitive dealings with the factions' enemy.

Ahmad al-Ahmad, a Syrian opposition activist who resides in Aleppo, said prices began going up a few days in opposition areas.

"We have no other option except to buy from Daesh... We need oil, and they need vegetables, so there is an exchange," he said. "We are now at the doors of winter, and we need fuel to keep warm and for our cars."

That underlines the problem in the campaign against IS oil infrastructure: Millions of civilians rely on it.

"The oil infrastructure is something that the civilian population benefits from. You don't want to punish people," Defense Secretary Ash Carter told MSNBC Thursday.

The campaign also underlines the limitations of an air campaign.

Smugglers will likely switch to small trucks, said Chistopher Harmer, senior naval analyst at the Washington-based Institute for the Study of War.

As a result, "it's going to become more and more difficult to differentiate between a pickup truck that is carrying crude oil and a pickup truck that just belongs to a baker or a farmer."

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George reported from Baghdad. Associated Press reporters Vladimir Isachenkov in Moscow and Robert Burns in Washington contributed to this report.