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.

联合国秘书长潘基文称，叙利亚未能按照一项和平计划的规定将部队与重型武器撤出市区。

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 envoy¹ Kofi Annan.

There have been continuing reports of violence in breach² of a ceasefire.

Syria had earlier said it would only accept 250 UN monitors.

Violence 'escalating³'

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 escalated⁴ again in recent days and "the cessation（停止） of armed violence in all its forms is therefore clearly incomplete".

He said the observers had been prevented from going to the city of Homs which has seen much of the violence in recent months.

The Syrian government cited "security concerns" for preventing their visit. But observers were allowed to move freely during their visit to Deraa.

Despite the violence and the gravity of the situation, Mr Ban also said "an opportunity for progress may now exist".

Mr Ban wants the observers deployed⁵ to 10 locations in stages over the coming weeks as conditions allow.

The BBC's Barbara Plett at the UN says Mr Ban hopes their presence on the ground will in itself help end the violence.

The UN Security Council meets later on Thursday to discuss Mr Annan's report. But the council is only expected to vote on a new resolution on Syria next week.

The Friends of Syria group of Syrian opposition⁶ groups and supporters is also meeting in Paris on Thursday.

Mr Annan's six-point peace plan calls for Syrian forces to withdraw from residential⁷ areas, the release of political prisoners, the permitting of peaceful demonstrations⁸, greater access for the media and the start of a democratic political transition.