DAMASCUS - US-led coalition warplanes on Sunday kept up their strikes against oil sites in Syria that fund the Islamic State group, as well as targeting the jihadists' command structure.
The air raids came a day after Al-Qaeda's Syria affiliate threatened reprisals after a key operative was reported killed.
The United States, along with coalition partners Saudi Arabia and the United Arab Emirates, hit four modular refineries and an IS command and control post, all north of Raqa in Syria, US Central Command said.
"Initial indications are that they (the strikes) were successful," it said.
The latest raids were part of intensifying efforts to deny IS funding after a wave of strikes on its oil infrastructure on Thursday night.
IS controls a swathe of territory straddling northwestern Iraq and eastern Syria, that includes most of Syria's main oilfields.
The coalition strikes hit close to the Turkish frontier, near Tal Abyad just across the border from the Turkish town of Akcakale, the Syrian Observatory for Human Rights said.
"At least three makeshift refineries under IS control in the Tal Abyad region were destroyed," it said.
"IS had been refining crude and selling it to Turkish buyers," said the Britain-based monitor, which relies on a network of sources inside Syria.
Before Tuesday's US-led air strikes on IS in Syria began, analysts say the jihadists were earning as much as US$3 million (s$3.8 million) daily from oil.
Coalition warplanes hit the jihadist heartland province of Raqa early Sunday as they pressed what Washington called "near continuous" strikes.
The raids also destroyed a plastics factory outside Raqa city, killing one civilian, the Observatory said.
IS oil infrastructure has been a main target of the bombing campaign in Syria launched by Washington and its Arab allies, building on the air war under way against IS in Iraq since August 8.
Sunday's air strikes also destroyed a tank and damaged another near Deir Ezzor in eastern Syria, US Central Command said.
Human Rights Watch, meanwhile, said apparent US missile strikes had killed at least seven civilians in Syria's northwestern Idlib province last Tuesday and called for a probe into possible violations of the laws of war.
The Pentagon said US-led air strikes in Iraq near insurgent-held Fallujah on Sunday destroyed two IS checkpoints and a transport vehicle.