Syria 'failing to keep to truce', says Ban Ki-moon
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 also called for an observer mission to be expanded from the small number already in the country to 300.
There are continued reports of violence in Syria, threatening a fragile ceasefire that began a week ago.
However, Mr Ban also said there was an "opportunity for progress".
Meanwhile, US Secretary of State Hillary Clinton will meet foreign ministers in Paris later to discuss the crisis.
The peace plan, negotiated by UN-Arab League envoy Kofi Annan, aims to end more than a year of unrest thought to have killed at least 9,000 people.
Expressing concern over continuing violence, Mr Ban said it was critical that the government kept its pledge to withdraw troops from populated areas.
He said while the level of violence dropped markedly when the ceasefire began, there had been an escalation in recent days, and breaches had been reported by both sides.
He also noted that there had been "no significant release of detainees", and "no substantive progress" in negotiations on humanitarian access, in reference to other aspects of Mr Annan's six-point plan.
He said observers had been prevented from going to the city of Homs after the government cited "security concerns". But observers were allowed to visit the town of Deraa freely.
The BBC's Jim Muir in neighbouring Lebanon says that while the trip to Deraa took place without incident, observers found themselves mobbed by protesters in the Damascus suburb of Arbeen.
Gunfire broke out and observers had to get out, he says. Several demonstrators were injured, according to activists.
In his report, Mr Ban said: "The situation in Arbeen became tense when a crowd that was part of an opposition demonstration forced United Nations vehicles to a checkpoint."
"Subsequently, the crowd was dispersed by firing projectiles. Those responsible for the firing could not be ascertained by the United Nations military observers."
The advance team of up to 30 monitors was authorised by a UN resolution passed on Saturday.
Mr Ban wants to increase the mission to 300 observers, to be deployed in 10 locations in stages over the coming weeks as conditions allow.
The UN Security Council is due to hold a closed-door meeting later on Thursday to discuss the report. A vote on a new resolution, however, is not expected until next week.
Activists said at least 30 people were killed on Wednesday, mostly in Homs where shelling has continued. Syrian state media on Thursday reported that four soldiers were killed in attacks in Aleppo.
Speaking ahead of a "Friends of Syria" meeting of foreign ministers in Paris, French President Nicolas Sarkozy called for humanitarian corridors to be established in Syria.
"[President] Bashar al-Assad is lying... He wants to wipe Homs off the map just like [former Libyan leader Muammar] Gaddafi wanted to destroy Benghazi," he said.
"The solution is the establishment of humanitarian corridors so that an opposition can exist in Syria," Mr Sarkozy told Europe 1 radio.
Russia said it was not attending the Friends of Syria talks, with a foreign ministry statement saying that the meeting was "not aimed at finding the grounds for dialogue within Syria, but quite the opposite".
A spokesman for the Chinese foreign ministry, Liu Weimin, meanwhile said it was "seriously considering" sending its own team of monitors.
In Turkey, port authorities are to search a ship after reports that it was carrying arms destined for Syria.
The Atlantic Cruiser, which is owned by a German firm but flies an Antigua and Barbuda flag, has docked at the Turkish port of Iskenderun.
German magazine Der Spiegel first reported last week that the cargo ship was carrying Iranian arms, which were loaded at a port in Djibouti, for Syria.