Damascus. Chemical weapons inspectors in Syria have been unable to access one of the designated sites because of safety concerns, the world’s chemical weapons watchdog has told the BBC.It is the first time they have been unable to complete a scheduled visit, the Organisation for the Prohibition of Chemical Weapons (OPCW) spokesman said.But overall the experts were pleased with their progress so far, he said.Their mission to rid Syria of chemical weapons was set up by a UN resolution.It followed international outrage at a chemical weapons attack near the Syrian capital Damascus in August. It is the first time the OPCW - which won this year’s Nobel Peace Prize - has been asked to oversee the destruction of a chemical weapons armoury during a conflict.The OPCW, which is based in the Dutch city of The Hague, said the team in Syria had completed nearly 50% of their work of inspecting more than 20 sites and destroying equipment.Under the UN resolution, Syria’s chemical weapons production equipment must be destroyed by 1 November and stockpiles must be disposed of by mid-2014.The deadline for Syria to submit its "destruction plan” was 15 November, the OPCW said on Thursday.But despite the good progress, inspectors were forced to turn back at one site after failing to receive assurances that they would be safe, OPCW spokesman Michael Luhan told the BBC.There had been a number of security incidents over the last few days, which had given them "cause for concern”, he said.On Wednesday night there had been a mortar attack near the hotel the inspectors are staying in in Damascus and over the weekend a number of improvised explosive devices (IEDs) were detonated in cars nearby, he said. Meanwhile, an international conference on a political solution to Syria’s conflict could take place in Geneva on 23-24 November, Qadri Jamil, Syria’s deputy prime minister, said on Thursday.He made the announcement after talks at the foreign ministry in Russia, Syria’s main international ally.More than 100,000 people have been killed in the fighting that has ravaged Syria for two-and-a-half years, according to the UN.