The Al-Nusra Front, Al-Qaeda's Syrian franchise, has threatened reprisals against nations participating in air strikes against the Islamic State group, denouncing them as "a war against Islam."

Group spokesman Abu Firas al-Suri said in a video posted online Saturday the states involved had "committed a horrible act that is going to put them on the list of jihadist targets throughout the world."

The warning came as the US-led coalition widened its air strikes against the IS group in Syria, as British warplanes flew their first anti-jihadist combat missions over neighbouring Iraq.

Washington has been supported in its Syria campaign by Arab allies Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates.

Seven targets were hit in Syria, the Pentagon said, including at the border crossing into Turkey of the besieged Kurdish town of Ain al-Arab, also known as Kobane.

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, identified as a member of Al-Qaeda, expressed condolences for the deaths of Fadhli and another Khorasan leader, Abu Yusuf al-Turki.

The US-based monitoring service said the jihadist in Twitter postings dated September 27, 2014, also lamented the situation on the ground in Syria as coalition forces bombard IS forces.

The United States and its coalition partners aim to destroy the Islamic State group, which controls a swathe of territory in Iraq and Syria, has murdered two US journalists and a British aid worker and is locked in a brutal war with Iraqi and Kurdish authorities.

The IS campaign there has already driven 160,000 refugees into Turkey.

Royal Air Force Tornado GR4 jets took off from Britain's RAF Akrotiri on Cyprus for Iraq but returned to base without dropping their laser-guided bombs.

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

Belgium and Denmark have also approved plans to join France and the Netherlands in targeting IS in Iraq, allowing Washington to focus on the more complex operation against its Syria base.

Washington warned that the jihadists could not be defeated in Syria by air power alone, saying that up to 15,000 "moderate" rebels would need to be trained.

- Turkey mulls 'necessary steps' -

President Recep Tayyip Erdogan said Turkey could take a military role in the coalition, the Hurriyet daily reported.

He said the government would go to parliament with a motion on October 2, after which "all the necessary steps" would be taken.

Ankara had previously insisted its hands were tied over dozens of Turkish hostages abducted by IS in Iraq, but they are now free.

Hundreds of Syrian Kurdish refugees, clutching whatever they could grab, crossed the border Saturday to safety.

Turkey's NTV television reported that shells fired from Syria hit Suruc, about 10 kilometres (six miles) north of the border, wounding two women.

Senior Syrian Kurdish official Newaf Khalil told AFP that air strikes hit the IS-held town of Ali Shar east of Ain al-Arab, destroying several IS tanks.

Coalition aircraft also pounded the Euphrates valley city of Raqa, which the jihadists have made the headquarters of the "caliphate" they declared in June over swathes of Iraq and Syria.

"At least 31 explosions were heard in Raqa city and its surroundings," said the Britain-based Observatory.

Washington has been keen not to let Syrian President Bashar al-Assad exploit the anti-IS campaign to make gains in the more than three-year-old civil war.

- 'Near continuous' combat sorties -

The US and Arab allies began air strikes against IS in Syria on Tuesday, more than a month after Washington launched its air campaign against the jihadists in Iraq.

Washington had been reluctant to intervene in Syria, but acted after the jihadists captured more territory and committed widespread atrocities, including beheading three Western hostages.

A US defence official told AFP Friday the Syrian mission is now similar to Iraq's, with "near continuous" sorties.

Washington also plans to train and arm 5,000 Syrian rebels, although top US military officer General Martin Dempsey said 12,000-15,000 men would be required to recapture "lost territory" in Syria.

Dempsey said defeating IS would take more than air strikes and that "a ground component" was an important aspect of the campaign.

At the UN General Assembly, Russian Foreign Minister Sergei Lavrov accused Washington of declaring its "right to unilateral use of force anywhere to uphold its own interests," in a veiled reference to the Syrian campaign.

European governments have so far ruled out strikes in Syria, although Britain "reserved the right" to intervene there if there was an imminent "humanitarian catastrophe".

Iran's ground forces commander General Ahmad Reza Pourdestanahas warned that it too would attack IS in Iraq if it approached the border, state media reported.

The Sunni extremists of IS control territory north of Baghdad, including in Diyala province bordering Shiite Iran.

In Diyala, the army said Shiite militias retook a dam after fighting believed to have killed dozens, security sources said.

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