US-led warplanes kept up strikes on oil sites funding the Islamic State group on Sunday, as Al-Qaeda's Syria affiliate threatened reprisals after a key operative was reported killed.

The coalition raids destroyed three makeshift oil refineries in jihadist-controlled territory in Syria, 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 oil fields and which the jihadists have sought to exploit through improvised refining and smuggling.

The coalition strikes hit close by the Turkish frontier, near the town of 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," the Observatory said.

"IS had been refining crude and selling it to Turkish buyers," said the Britain-based watchdog, which has a broad network of sources inside Syria.

Before the launch of US-led air strikes on IS in Syria last Tuesday analysts say the jihadists were earning as much as $3 million a day from oil revenues.

Output from IS-controlled fields stood at 80,000 barrels per day, far exceeding the 17,000 barrels per day the Syrian oil ministry said it was pumping.

The strikes around Tal Abyad came after Saturday raids near the mainly Kurdish town of Ain al-Arab, also very close to the Turkish border.

The town, known as Kobane in Kurdish, has been under assault by IS for more than a week, sparking an exodus of at least 160,000 refugees into Turkey.

The coalition also kept up its raids on the jihadist heartland province of Raqa early Sunday as it pressed what Washington says are "near continuous" strikes.

The raids destroyed a plastics factory outside Raqa city, killing one civilian, the Observatory said.

IS oil infrastructure has been one of the main targets of the bombing campaign in Syria that Washington and its Arab allies launched last Tuesday, building on the air war under way against IS in Iraq since August 8.

Multiple European governments have since approved plans to join that air campaign, including most recently Britain.

- First British mission -

British fighter jets flew their first combat mission over Iraq on Saturday but returned to base in Cyprus without firing a shot.

"On this occasion no targets were identified as requiring immediate air attack by our aircraft," a defence ministry spokesman said.

European governments have resisted joining the US-led air campaign in Syria for fear of getting embroiled in the more than three-year-old civil war, forcing Washington to rely on Arab allies Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates.

The opening salvo of the US-led bombing campaign in Syria actually targeted not IS but its jihadist rival Al-Qaeda and drew a threat of retaliation on Saturday after one of its leaders was reported killed.

Al-Qaeda loyalists of Al-Nusra Front, who are prominent on the battlefield across much of western Syria, have been at sometimes deadly loggerheads with IS since the Al-Qaeda leadership disavowed the group's commanders in February.

But the group has still been targeted by the US-led air campaign which has killed at least 57 of its fighters, according to 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 Qaeda operative and alleged leader of Khorasan, was killed in the strikes, according to a jihadist who fought with the group.

The SITE monitoring group said a series of Tweets from the jihadist expressed condolences for the deaths of Fadhli and another Khorasan leader, Abu Yusuf al-Turki.

But Al-Nusra said it was the target of the strikes and threatened reprisals for the deaths of its militants.

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 said in an online video message.

Al-Nusra is on Washington's terror blacklist but has fought against IS in several areas of Syria, notably around second city Aleppo where its fighters were instrumental in driving out the rival jihadists in alliance with other Islamists.