BEIRUT/PARIS (Reuters) - Gunfire and protests near a team of U.N. observers in Syria and diplomatic wrangling over the scope of their mission underlined the precarious prospects for a week-old truce which has so far failed to halt a year of bloodshed.
Shooting erupted on Wednesday close to an advance party of military personnel from the United Nations who had been swarmed by protesters denouncing President Bashar al-Assad in the town of Erbin, on the northeastern outskirts of the capital Damascus.
There were no reports of casualties. But scenes of monitors' vehicles stuck in a crowd and men running away while gunfire rattled in the air were an ominous echo of an earlier monitoring mission, by the Arab League, which collapsed in January.
The United States said the incident - and sustained shelling by the Syrian army of the city of Homs - raised questions about whether observers could actually monitor any truce and pledged to step up pressure on Assad at a meeting in Paris on Thursday.
No formal agreement has been reached yet on how the new monitors should operate, and Syria challenged U.N. Secretary Ban Ki-moon over the size and scope of their mission.
Ban said this week that 250 observers were insufficient in a country of 23 million where the United Nations says at least 9,000 people have been killed in the past 13 months. He sought European help in supplying planes and helicopters.
But 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.
He also dismissed any need for U.N. aircraft.
At the same time, the Syrian army kept up its shelling of targets in Homs in violation of the ceasefire deal worked out with international envoy Kofi Annan. Explosions shook the battered Khalidiyah quarter and plumes of black smoke drifted over the rooftops.
To the north, in Idlib province, six members of the security forces were killed by a bomb placed by an "armed terrorist group", the state news agency SANA said. It was the second such attack in two days. Syria bars access to most independent journalists, making it hard to verify accounts of the conflict.
"We are at a crucial turning point," U.S. Secretary of State Hillary Clinton said at a meeting of NATO ministers in Brussels.
"Either we succeed with ... the Annan plan with the help of monitors ... or Assad will squander his last chance before additional measures have to be considered."
While the truce has held in some parts of Syria, in strong opposition areas such as Homs, Hama, Idlib and Deraa the army has kept up attacks on rebels, using heavy weapons in violation of the pledge by Damascus to Annan to pull back.
Clinton will join Arab and European foreign ministers from the informal Friends of Syria group in Paris on Thursday.
"We will continue to increase the pressure on Assad," she said, adding that she had spoken to counterparts about the need to "tighten sanctions, tighten pressure on the regime, on those who support the regime".
The United States and European Union already have extensive economic sanctions on Syria. But the Arab League, which announced a package of financial sanctions against Damascus in November, has done little to enforce the proposals.
Two previous Friends of Syria meetings, in Tunisia and Turkey, produced more rhetoric than results and it was not clear what Thursday's smaller gathering in France might deliver.
Clinton, as she has in the past, appeared to leave the door open to other nations arming Syrian rebels - something the United States has itself rejected although it is giving the opposition communications and logistical assistance.
Washington and its Western allies have shown no desire to intervene militarily or push for the sort of robust peacekeeping force in Syria that might require 50,000 troops or more.
The Syria mission was negotiated by Annan, a former U.N. secretary-general now acting as an envoy of the United Nations and Arab League. Diplomats say Annan's main aim is to get a U.N. mission on the ground backed by Syria's supporters Russia and China, even if it is not big enough at first to do the job.
The U.S. ambassador to the United Nations, Susan Rice, said it was not clear if conditions would allow the observer mission to deploy after the difficulty met by the advance party, led by Colonel Ahmed Himmiche of Morocco.
"The (Security) Council was clear there needs to be a sustained cessation of violence, there has to be the ability for this advance contingent to move freely and unimpeded, and I think that there's reason on both counts to be concerned that thus far those conditions are not in place," Rice said.
Assad says Syria is under attack by foreign-backed terrorists who have killed 2,500 soldiers and police, and that for their own safety the unarmed observers would have to coordinate every step of their operation with Syrian security.
The rebel Free Syrian Army fighting to topple Assad says it will stop shooting if he keeps his unfulfilled pledge to Annan to withdraw tanks, heavy weapons and troops from urban areas.
Apart from the shelling of targets in Homs, the city at the heart of the revolt, troops have swept towns and villages in raids to arrest suspected opponents of Assad.
Activists say scores of people have been killed since the ceasefire officially came into force last Thursday.
Writing by Dominic Evans; Editing by Alastair Macdonald