The world's chemical weapons watchdog said Thursday that it had completed nearly half its inspections of Syria's arsenal with a view to its destruction by mid-2014.

"We have done nearly 50 percent of the verification work of the facilities that have been declared to us," Malik Ellahi, a political advisor on Syria for the Organisation for the Prohibition of Chemical Weapons, told journalists in The Hague.

Despite the progress, Ellahi said security remained a concern for the unprecedented mission in war-torn Syria, with mortar and car bomb attacks around the inspectors' Damascus hotel.

"One of the things that is of concern is of course the security situation," said Ellahi, who advises OPCW Director-General Ahmet Uzumcu.

"There have been a number of incidents over the last few days which gives some cause for concern."

The OPCW said on Wednesday that its inspectors had checked 11 out of 20 sites identified by Damascus and destroyed chemical weapons equipment at six sites.

The organisation, which last week was awarded the Nobel Peace Prize, and the United Nations currently have about 60 experts working in Syria to eradicate chemical weapons, around a month after the OPCW accepted President Bashar al-Assad's application to join the Chemical Weapons Convention, in a bid to stave off a possible Western military strike.