Cairo, Sep 27 (IANS/EFE): The US-led international coalition carried out airstrikes Saturday on Islamic State (IS) positions in the city of Raqqa, the Sunni militants' main stronghold in Syria.

The Britain-based Syrian Observatory for Human Rights, which monitors the conflict in Syria through a network of contacts in that country, said in a statement that roughly 30 explosions were heard in that northern city and its outskirts.

The missiles struck several IS camps and a government building, the observatory said.

The IS, a coalition of jihadis, tribal militias and veterans of the late Saddam Hussein's army, has proclaimed a caliphate in the parts of Syria and neighbouring Iraq under its control.

The US military and Arab allies Saudi Arabia, the United Arab Emirates, Jordan and Qatar launched their first airstrikes against IS positions in Syria late Monday, the US Defence Department said the following day.

The attacks this week have mainly targeted IS bases and positions in the northeastern Syrian provinces of Deir al-Zur, al-Hasakah and Raqqa.

IS-controlled oil installations have been among the main targets of the airstrikes in Deir al-Zur, which borders Iraq.

Separately, Britain's defence ministry said Saturday that two British warplanes loaded with laser-guided bombs and missiles began their first mission in Iraq against the Sunni militants and "are now ready to be used in an attack role as and when appropriate targets are identified".

British lawmakers voted Friday by an overwhelming majority to authorise the government to carry out airstrikes against IS targets in Iraq.

Friday's outcome was a striking turnaround from a year earlier, when parliament blocked Prime Minister David Cameron's bid to carry out attacks in Syria after the Assad regime's alleged use of chemical weapons.

The motion did not authorise British airstrikes in Syria, but Cameron left the door open, insisting that there were no "legal barriers" to extending attacks to Syria in the future.