BEIRUT, Lebanon: Air strikes believed to have been carried out by US-led forces hit three makeshift oil refineries in Syria’s Raqqa province early on Sunday as part of an assault to weaken Islamic State (IS) militants, a monitoring group said.

The Britain-based Syrian Observatory for Human Rights said the attacks occurred shortly after midnight, adding that they also hit a plastic factory.

Islamic State fighters have control over oil produced in eastern Syria and have set up small, makeshift refineries to distil the crude into fuel, one of their main sources of income.

The United States and five Arab allies launched an aerial campaign against Islamic State fighters Tuesday with the aim of rolling back and ultimately crushing the extremist group, which has created a proto-state spanning the Syria-Iraq border.

In seizing territory, the Islamic State group has chased out tens of thousands of Syrians and Iraqis who belong to minority groups and has imposed their harsh interpretation of Islamic law on residents, including whipping and chopping off the hands of criminals.

US Central Command, which is overseeing the operation against the Islamic State group, did not immediately comment on the strikes. A US-led coalition has been targeting oil installations controlled by the militants, aiming to crippling the Islamic State group’s finances, estimated to earn some $3 million a day.

But the impact of the strikes on the group’s capabilities was not immediately clear.

“These so-called refineries are not a real target and they do not weaken the Islamic State as they do not have any financial value for them,” Rami Abdelrahman of the Observatory told Reuters.

“They are composed of trucks with equipment to separate diesel and petrol used by civilians.”

IS has gained sympathy from many Islamists after the attacks, including from rival groups.

Scores of fighters have left Al-Qaeda’s Nusra Front and other Islamist groups in Syria to join IS since the strikes started.

The Nusra Front is coming under increasing pressure from its own members to reconcile with IS to fight what they describe as a “crusader” campaign against Islam.

On Saturday, the group gave its first reaction to the strikes, vowing to retaliate against Western and Arab countries that took part in the coalition.

“It’s not a war against Nusra Front, it’s a war against Islam,” Nusra spokesman Abu Frias Al-Suri said in an audio message published on the group’s social media network.

“These countries have done a despicable act that will put them on the list of those targeted by jihadist forces all over the world,” the spokesman said.