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 $3 million (2.4 million euros) 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.On the ground in western Iraq, pro-government forces backed by warplanes on Sunday repelled an IS attack on the strategic town of Amriyat al-Fallujah, security sources said.
"Warplanes eventually engaged the insurgents and killed 15 of them," local police chief Aref al-Janabi said, without identifying the aircraft.
The town "has strategic importance. It is a main logistics road for the army and it is the link between Anbar and Karbala", a Shiite holy city south of Baghdad, Janabi said.
Several European governments have approved plans to join the air campaign in Iraq, including most recently Britain, while Turkish President Recep Tayyip Erdogan said Ankara "cannot stay" out of the fight.
British warplanes flew their first combat mission over Iraq on Saturday but returned to base in Cyprus with their bombs after no targets were identified.
"They are flying daily over the area where the fighting is and they are ready as part of the international force to be called in by the ground troops if they can help directly with some of the fighting," said Defence Secretary Michael Fallon.
European governments have resisted joining the US-led air campaign in Syria for fear of getting embroiled in the country`s more than three-year-old civil war, forcing Washington to rely on Arab allies.
On Sunday President Barack Obama admitted that the United States underestimated the IS threat in Syria, while overestimating the ability of Iraq`s US-trained military to fight the jihadists on its own.The opening salvo of the US-led bombing campaign in Syria actually targeted not IS but its jihadist rival Al-Nusra Front and drew a threat of retaliation after one of its leaders was reported killed.
Al-Nusra, the Syrian branch of Al-Qaeda, has been targeted by the US-led air campaign, killing at least 57 of its fighters, said the Observatory.
Washington has made a distinction between the wider Al-Nusra Front and a cell of foreign fighters dubbed the Khorasan Group that it says was plotting attacks against the United States.
Muhsin al-Fadhli, a long-standing Al-Qaeda operative and alleged leader of Khorasan, was reportedly among those killed in the strikes.
The allies had "committed a horrible act that is going to put them on the list of jihadist targets throughout the world," Al-Nusra spokesman Abu Firas al-Suri warned in an online video message.
Meanwhile, Germany began training Kurdish fighters from northern Iraq at an army school in Bavaria on handling weaponry from Berlin to support the battle against IS.