The UN secretary-general says Syria has failed to comply with its obligation under a peace plan to pull troops and heavy weapons out of urban areas.
In a letter to the UN Security Council, Ban Ki-moon called for an observer mission to be expanded to 300 members.
An advance team of six observers has already gone to Syria under the plan, negotiated by UN envoy Kofi Annan.
There have been continuing reports of violence in breach of a ceasefire agreed after a UN resolution.
Syria had earlier said it would only accept 250 UN monitors.
Mr Ban said although the ceasefire agreed by both sides following a UN Security Council resolution was incomplete, both sides said they were committed to ending the violence.
He said the level of violence dropped markedly on 12 April, the day the ceasefire called for by Mr Annan came into effect.
But he said violent incidents and reported casualties had escalated again in recent days and "the cessation of armed violence in all its forms is therefore clearly incomplete".
Despite the violence and the gravity of the situation, Mr Ban also said "there is a chance for progress".
The UN Security Council meets later on Thursday to discuss Mr Annan's report.
The Friends of Syria group of Syrian opposition groups and supporters is also meeting in Paris on Thursday.
The BBC's Barbara Plett at the UN says Mr Ban wants the observers deployed in stages over the coming weeks, as the conditions allow.
She says Mr Ban hopes their presence on the ground will in itself help end the violence.
Mr Annan's six-point peace plan calls for Syrian forces to withdraw from residential areas, the release of political prisoners, the permitting of peaceful demonstrations, greater access for the media and the start of a democratic political transition.