The head of UN monitors in Syria said on Thursday the mission must stay, despite the fact that the ceasefire they were sent to police is non-existent and violence is reaching an "unprecedented level".

General Robert Mood said the 300-strong mission should be restructured to help support the political dialogue that foreign powers say is the only way out of the crisis.

"Now we are in a situation where we have the contacts and knowledge, but we have no ceasefire. So it is time to stop spreading ourselves out too thin and restructure in a way that will allow us, once we resume our activities, to conduct targeted tasks that require longer periods of stay in particular areas," Mood told a news conference in Damascus.

Diplomats at the United Nations in New York said last month they were considering scaling down or eliminating the unarmed observer force, whose mission is currently suspended, if the bloodshed was not significantly reduced.

Activists say more than 15,000 people have been killed since the revolt against President Bashar al-Assad began in March 2011. In recent weeks, they have reported death tolls of around 100 a day.