The Syrian branch of al-Qaeda has promised retaliation over air strikes in Syria, as the US-led coalition widened its assault on ISIL targets and British jets flew their first combat missions over neighbouring Iraq.

In its first reaction to the US-led military operation in Syria, the Nusra Front said the air strikes were a "war against Islam", and threatened to attack the worldwide interests of participating Western and Arab countries.

A US attack on a Nusra base in Aleppo on the first day of the air campaign killed dozens of the group's fighters.

Nusra - listed as a terrorist group by the US and the UN - is one of the most powerful groups fighting the Syrian regime. It has also declared ISIL its sworn enemy.

In a video posted online on Saturday, a Nusra spokesman threatened the coalition partners.

"These states have committed a horrible act that is going to put them on the list of jihadist targets throughout the world," Abu Firas al-Suri said. "This is not a war against al-Nusra, but a war against Islam."

The US has been supported in its Syria air campaign by Arab allies Bahrain, Jordan, Qatar, Saudi Arabia and the UAE.

The warning came on a day the Pentagon said seven targets were hit in Syria, including at the border crossing into Turkey of the Kurdish town of Ain al-Arab, called Kobane by Kurds, which has been under siege by fighters of ISIL.

The US Central Command said an ISIL building and two armed vehicles were destroyed in the strikes.

ISIL's campaign in the area has driven 160,000 refugees into Turkey and hundreds more, clutching whatever they could grab, crossed the border on Saturday.

Al Jazeera's Stefanie Dekker, reporting from the Turkish side of the border, said the Kurds who had left their homes in Syria seemed to be disappointed by the US-led air campaign.

"Many of the people we have spoken to do not see the usefulness of the strikes as the ISIL continues to push into their areas," she said.

Hit by ISIL rockets

The UK-based Syrian Observatory for Human Rights monitoring group said ISIL rockets hit Ain al-Arab after the strikes, for the first time since the group's assault began on September 16, wounding 12 people.

As part of Saturday's assault, coalition aircraft targeted the Euphrates valley city of Raqqa, which ISIL fighters have made the headquarters of the "caliphate" they declared in June over a vast area comprising parts of Iraq and Syria.

The US and Arab allies began air strikes against ISIL and in Syria on Tuesday, more than a month after the Pentagon launched an air campaign against the self-declared jihadists in Iraq.

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

More countries, including the UK, are involved in strikes in Iraq.

On Saturday, jets took off from Britain's RAF Akrotiri on Cyprus for Iraq but returned to base without dropping their laser-guided bombs.

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

In a related development, Turkish President Recep Tayyip Erdogan said Turkey could take a military role in the coalition, the Hurriyet daily reported on Saturday.

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