Makeshift oil refineries in Syria, set up by the Islamic State, were hit during U.S.-led airstrikes Sunday, Reuters reported.

The Britain-based Syrian Observatory for Human Rights, a monitoring group that reported the attacks, which occurred shortly after midnight in Syria's Raqqa province, said the strikes were also successful in hitting a plastic factory

The news of the oil refineries is not significant, the group said, even though the Islamic State fighters have control over oil produced in eastern Syria and the makeshift refineries, to distill the crude into fuel, are one of their main income sources.

Rami Abdelrahman of the Observatory told Reuters the refineries are not the real target because, "They are composed of trucks with equipment to separate diesel and petrol used by civilians."

A medium-sized makeshift refinery, which can cost up to $230,000 to set up, can produce up to 200 barrels of crude a day. The refineries are assembled in Turkey and transported into Syria.

But an assessment of the true damage to IS from the destroyed refineries is not known yet.

What is known is that the airstrikes are increasing sympathy for the IS by many Islamists, including rival groups like the Nusra Front, whose fighters have left the al-Qaida group to join IS in their "crusader" campaign.

The attacks have angered the Nusra Front, who have vowed to retaliate against the coalition carrying out the attacks, which includes Western and Arab powers.

Despite the threat, the attacks have failed to significantly stop IS fighters advancement into Syria.

Kobani, near the border with Turkey, had seen about 150,000 refugees flee after being seized by IS fighters.

About 300-400 Syrian Kurds crossed back into Syria on Sunday from Turkey to help fight against IS, a soldier on the border told Reuters.

But Turkish authorities were not allowing them to go to Kobani, where some of the Turkish Kurds want to go fight.

"IS is evil. Of course, we want to go and fight (in Kobani)," Mustafa Durdu, a Turkish Kurd who has not been permitted to leave Turkey by Turkish authorities, told Reuters. "Our brothers are there."