Syrian rebel groups have chosen a former officer to head a new Islamist-dominated command.

The western-backed effort is to put the opposition's house in order as President Bashar al-Assad's army takes hits that could usher his downfall.

In Turkey, a newly formed joint command of Syrian rebel groups chose Brigadier Selim Idris, one of hundreds of officers who have defected from Assad's army, as its head.

Mr Idris, whose home province of Homs has been at the forefront of the Sunni Muslim-led uprising, was elected by 30 military and civilian members of the joint military command.

Talks were attended by Western and Arab security officials in the Turkish city of Antalya.

The unified command includes many with ties to the Muslim Brotherhood and to Salafists, who follow a puritanical interpretation of Islam.

It excludes the most senior officers who have defected from Assad's military.

On the Damascus battlefront, Mr Assad's forces used multiple rocket launchers on today against several suburbs that have fallen to rebels who have fought their way to the edge of the city's international airport, where foreign carriers have suspended all flights.

Rebels, who have overrun several army bases near Damascus over the last month, appeared to be holding their ground, encircling a main military base in the northeastern suburb of Harasta.

The military base is known as "idarat al markabat", near the main highway to Aleppo, according to opposition campaigners.

Meanwhile, British Foreign Secretary William Hague has said he had seen "some evidence" that Syria is preparing to use chemical weapons against rebels.

Mr Hague declined to give details of the intelligence, but again warned that the regime of the Syrian President Bashar al-Assad would face action if such weapons were deployed.

United States satellites and other tools have reportedly detected increased activity at several chemical weapons depots in Syria.

At least one military base is also said to have been ordered to begin combining components of Sarin nerve gas to make it ready to use.

The Syrian regime has denied any plans to use chemical weapons against it own people.