Jakob Kellenberger, president of the International Committee of the Red Cross, said that while he is still hopeful, "unfortunately I am also aware how much this plan is at risk".
"That's why it's especially important for this mission to expand quickly," Mr Kellenberger told the Swiss newspaper Der Sonntag. He met with Syrian leaders earlier this month.
Syria's Deputy Foreign Minister Faisal Mekdad has criticised the UN, claiming that the government are being 'unfairly targeted'.
Major General Robert Mood is the man tasked with leading the mission to observe a ceasefire in Syria.