guardian.co.uk — President Assad says he will need written agreement over violence in move observers say is designed to divide oppositionUN-backed plans for a ceasefire and negotiations in Syria have been thrown into doubt by a new demand from President Bashar al-Assad that armed groups guarantee to halt all violence before regime forces are withdrawn from flashpoints across the country.Kofi Annan, the former UN secretary-general, had called for a pullout to be completed on Tuesday with a full ceasefire 48 hours later.But after days of intense violence, Damascus insisted on Sunday that it would first need written guarantees from the opposition.