The United States, joined by Saudi Arabia and the United Arab Emirates, resumed bombing raids against Islamic State jihadists in Syria on Wednesday, with warplanes targeting oil refineries held by the group, the Pentagon said.

The US-led air strikes for the first time hit oil installations in eastern Syria in a bid to undercut a key source of income for the IS group, which relies on sales from smuggled crude oil to middlemen across the region.

US fighter jets on a degrading mission

The latest round of air raids focused on 12 targets in eastern Syria, where the IS extremists control small-scale oil refineries, Pentagon spokesman Rear Admiral John Kirby told CNN.

“These 12 targets were what we call modular oil refineries,” Kirby said.

“They were struck with precision-guided missiles by coalition aircraft. In fact, there were more coalition aircraft in the skies on these particular missions than US (planes),” he said.

Kirby confirmed that aircraft from Saudi Arabia and the United Arab Emirates took part in the latest raids.

The well-funded IS extremists have seized several oil fields in Syria and rudimentary refineries, enabling them to sell smuggled crude oil at cheap prices through intermediaries in Turkey, Iraq, Iran and Jordan.

Analysts say the group operates in a similar manner to a mafia and uses kidnapping ransoms, extortion and robbery — in addition to oil smuggling — for its funding.

The United States launched an air campaign against the IS group in Iraq last month and expanded the strikes into Syria early Tuesday, with five Arab countries joining a coalition effort.

The precise contribution of Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates has not been revealed but US military officers say warplanes from a number of the countries have dropped bombs on IS positions in Syria.

Jordanian aircraft conducted an air strike overnight in Syria, officers sa