Syria and the United Nations signed an agreement on Thursday on terms for hundreds of observers to monitor a ceasefire, but fierce diplomatic wrangling lies ahead to persuade the West the mission can have the authority and power to ensure peace.

An handful of U.N. observers are already in Syria monitoring a week-old truce that has failed to stop bloodshed. The question of whether the mission can expand while violence continues is up in the air. A crowd mobbed the head of the advance party on Thursday, some demanding the death of President Bashar al-Assad.

The 15-member U.N. Security Council - divided between Western countries that want to topple him and Russia and China which support Assad - must agree the proposal to send a larger observer force.

Senior officials from Western states including France, the United States and Britain, met in Paris with Middle East powers including Turkey, Qatar and Saudi Arabia. They agreed the truce was the last hope of avoiding an all-out civil war. But Russia snubbed an invite and derided the Friends of Syria meeting.

U.S. Secretary of State Hillary Clinton told the group in Paris that they should push for a Security Council resolution to impose U.N. sanctions on Assad if he blocked an adequate peace monitoring mission. She said Russia, while still likely to veto such a measure now, might support one if violence went on.

In the first progress report since the council authorised the arrival of the initial observers on Saturday, U.N. Secretary-General Ban Ki-moon said Syria had not fully withdrawn troops and heavy weapons from towns as agreed, failing to send a clear signal about its commitment to peace.

On Thursday, the council was briefed by mediator Kofi Annan's deputy, Jean-Marie Guehenno. According to council diplomats, Guehenno acknowledged risks of deploying unarmed observers while violence persists, but said their presence could help by changing the political dynamics on the ground.

FIGLEAF FEARS

Assad's opponents fear that a small observer mission with a weak mandate would act as little more than a fig leaf for the government, blocking more robust intervention to halt a 13-month crackdown on cities that have risen up against Assad.

U.S. and European diplomats on the council have suggested Syria's lack of full compliance with the ceasefire might make it hard for them to back an expanded mission. Russia and China will approve a mission only under a part of the U.N. charter that gives Assad's government a veto over the observers' mandate.

Discussion has focused on a force of at most a few hundred people, a tiny fraction of the size of peacekeeping forces normally deployed to war zones.

This preliminary agreement ... aims to facilitate the task of the observers within the framework of Syrian sovereignty, a statement from the Syrian Foreign Ministry said.

The small U.N. advance team on the ground in Syria has already had a taste of the unrest close up. On Wednesday, gunfire erupted close to the observers, who had been swarmed by anti-Assad protesters near Damascus.

On Thursday the team went to a rural area near the town of Deraa, where the uprising against Assad began 13 months ago. Amateur video footage posted on the Internet showed the team's head, Colonel Ahmed Himmiche, wearing a U.N.-style turquoise bullet-proof vest as he walks through a crowd of protesters.

A demonstrator wraps his arm around Himmiche and shouts: The people want the execution of Bashar, as Himmiche, who must act as a neutral observer, looks ill at ease.

NO AIRCRAFT

The last mission by outside monitors to observe a peace plan in Syria, sent by the Arab League, collapsed in failure in January after just a month. That team said it could do little as long as its movements were controlled by Assad's forces.

Ban has asked for any new U.N. mission to have its own aircraft so it can travel independently, but Syria says it will provide transport. Edmond Mulet, deputy head of the U.N. Department of Peacekeeping Operations, told the Security Council U.N. air assets were not covered in the preliminary agreement, council envoys told Reuters on condition of anonymity.

The anti-Assad Syrian Observatory for Human Rights said Syrian forces opened fire in the town of Herak, in southern Syria, shortly after the U.N. team left the area on Thursday.

It added that six people had been killed around Syria on Thursday, including two during army shelling in Homs.

Ban has proposed an expanded mission of 300 personnel to monitor the shaky ceasefire.

Syrian Foreign Minister Walid al-Moualem said 250 was a reasonable number, adding they should be from countries such as China, Russia, Brazil, India and South Africa - all more sympathetic to Damascus than are the West or the Arab League.

Chinese Foreign Ministry spokesman Liu Weimin said Beijing was seriously studying participating.

RUSSIA SCOLDS FRIENDS OF SYRIA

The informal Friends of Syria group said in a statement in Paris: Every day that passes means dozens of new Syrian civilian deaths.

It is not time to prevaricate. It is time to act... Though fragile, the Annan mission represents a last hope.

French Foreign Minister Alain Juppe told the meeting: We cannot wait, time is short ... The observers must be deployed fast and must be able to act without obstacles.

Clinton told the meeting: We have to do more to take tougher actions against the Assad regime. We need to start moving very vigorously in the Security Council for a Chapter VII sanctions resolution, including travel, financial sanctions, an arms embargo, and the pressure that that will give us on the regime to push for compliance with Kofi Annan's six-point plan.

Chapter VII of the U.N. charter gives the right to use force to ensure compliance, although for now the Western powers have no appetite for military intervention. Russia, angered by a U.N. resolution that led to war in Libya last year, opposes sanctions and says the West and Arab powers are failing to give due weight to Assad's argument that he is fighting Islamist militants.

When the so-called Syrian group of friends meet and somebody says 'Now we'll assess how Assad implements Kofi Annan's plan', it is a wrong attempt, Russian Foreign Minister Sergei Lavrov said at a NATO meeting in Brussels. We cannot privatise (the plan) and we will not let it happen.

Clinton said of her talks with Lavrov in Brussels: He was, as usual, very intent upon laying responsibility on all sides, and in particular on the opposition. But he also has recognized that we are not in a static situation but a deteriorating one.

French President Nicolas Sarkozy, facing the first round of an uphill re-election battle on Sunday, said the solution for the crisis in Syria would be to set up a humanitarian corridor that would allow the opposition to Assad to survive.

Assad wants to wipe Homs off the map just like Gaddafi wanted to destroy Benghazi, said Sarkozy, whose lead in backing Libya's rebels against Muammar Gaddafi last year won him praise at home and abroad. We called this meeting to gather all those who cannot stand that a dictator is killing his people.

On Wednesday, 22 people were killed in Syria, including 13 civilians during shelling in Homs, said the British-based Syrian Observatory for Human Rights, which compiles information from anti-Assad activists. Syria's state news agency said 13 people were killed by armed groups, including six law enforcement officials after a roadside bomb.

In Ban's report, the secretary-general expressed some hope that there may be a chance for progress on ending the conflict.

He said the advance team enjoyed freedom of movement in Deraa, but its initial request to visit Homs, a centre of the uprising against Assad, had been refused.

Activists posted footage on the internet of further shelling in Homs city on Thursday.

(Additional reporting by Dominic Evans, John Irish and Daniel Flynn in Paris, Tom Miles in Geneva, Ben Blanchard in Beijing, Sebastian Moffet and Arshad Mohammed in Brussels; Writing by Oliver Holmes; Editing by Peter Graff and Alastair Macdonald)