Syria has surrendered more than half of its chemical weapons arsenal, the joint mission overseeing the dismantling of the banned stockpile said on Thursday."As of today 53.6 percent of the Syrian Arab Republic's chemical weapons material has been removed from or destroyed in the Syrian Arab Republic," the joint mission said.The mission of the United Nations and the Organisation for the Prohibition of Chemical Weapons said the milestone represented "important progress.""The Joint Mission welcomes the momentum attained, and encourages the Syrian Arab Republic to sustain the current pace," it said in a statement.Syria remains behind schedule for the destruction of its entire arsenal by June 30, a deadline agreed by Russia and the United States last year.The deadline came as part of a plan to avert US-backed military strikes in the wake of deadly chemical attacks outside Damascus last year blamed by the West on President Bashar al-Assad's regime.But the OPCW earlier this month noted that Syria had increased the pace of removing or destroying its arsenal after missing critical deadlines.