* Syria says 250 observers will be enough
* Says mission will not need its own helicopters
* Shooting near U.N. advance team
* Ban Ki-moon says bigger operation needed
BEIRUT, April 18 (Reuters) - Gunfire erupted on Wednesday close to an advance team of U.N. observers who had been swarmed by protesters, giving a taste of the challenge facing a mission to monitor a shaky week-old truce that has so far failed to stop Syria's violence.
Protesters denouncing President Bashar al-Assad had surrounded their cars near the capital, Damscus. Automatic weapons fire sent the crowds scurrying for safety.
There were no reports of casualties. But scenes of monitors' vehicles immobilised in a crowd followed by pictures of men running away while automatic weapons fire rattles in the air were an ominous echo of an earlier Arab League monitoring mission that collapsed in failure in January.
The advance party of a half a dozen U.N. peacekeepers in blue berets, led by Colonel Ahmed Himmiche of Morocco, was touring the vicinity of Damascus in two white U.N. Land Cruisers with a Syrian police escort when trouble began.
Their cars were mobbed by protesters chanting demands to arm the rebel Free Syrian Army. A banner was plastered on one U.N. car reading: "The butcher continues killings. The observers continue observing, and the people continue with their revolution. We bow only to God."
A video segment posted on YouTube showed the U.N. vehicles moving slowly down a street flanked on both sides by crowds of civilians when gunshots sounded somewhere up ahead and the men turned and ran for safety.
U.N. Secretary-General Ban Ki-moon has said that a team of just 250 observers is not big enough. But on Wednesday Syria - which has the power to approve the force under a U.N. Security Council resolution - rejected Ban's call for more monitors.
At the same time, the army kept up its shelling of targets in the city of Homs in violation of the week-old ceasefire.
Explosions rocked the battered Khalidiyah quarter of Homs and plumes of black smoke drifted over the rooftops.
In northern Idlib province, six members of the security forces were killed by a bomb placed by an "armed terrorist group", state news agency SANA said. It was the second such attack in two days.
"We are at a crucial turning point," U.S. Secretary of State Hillary Clinton said. "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 since Assad pledged to enforce it last week, 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 pull back.
Syrian Foreign Minister Walid al-Moualem told a news conference in Beijing that no more than 250 truce monitors were needed, and they should come from what he called "neutral" countries such as Brazil, Russia, India, China and South Africa, all of which have been more sympathetic to Assad than the West and the Arab League states.
The U.N. chief Ban was due to present proposals for the next phase of the mission on Wednesday to the Security Council. He says more monitors and aircraft are needed to supervise a truce in a country of 23 million, after 13 months of fighting that has killed more than 10,000 people.
The earlier Arab League monitoring mission was aborted in January after just a month in country because of daily shooting and shelling. During its short life it emboldened Syrians to resume anti-Assad street protests - an effect the government will not want to see repeated.
With the flashpoint cities in Syria scattered over several hundred kilometres, Ban said he had asked the European Union if it could supply helicopters and planes to make the proposed U.N. monitoring mission independently mobile. But Moualem said any needed air transport would be supplied by Syria.
A political source in neighbouring Lebanon said Damascus has already refused the use of U.N. helicopters. The mission is empowered under an article in the U.N. charter that requires the Syrian government's consent.
Video of the U.N. monitors filmed by activists showed mission leader Himmiche walking through a tight throng of demonstrators, wearing a U.N. blue beret and flak-jacket. He got into his vehicle where he spoke on a loud-hailer, apparently asking the crowd to move back and let the cars move out.
Any shots were not aimed at the observers, he later told Reuters TV: "We did not come under fire."
Equipment for the mission, including vehicles, is already being transported to Syria via Beirut from a U.N. logistics base in Brindisi, Italy and Prague in the Czech Republic. Four C-130 transports have flown 10 4x4 vehicles to the Lebanese capital for delivery to the mission.
The mission was negotiated by former U.N. Secretary-General Kofi Annan, 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 West has shown no desire to intervene militarily or push for the sort of robust peacekeeping mission that might require 50,000 troops or more.
Russia and China, Syria's Security Council veto-wielding friends, have made clear they would block a U.N. mandate to use force. They are likely to back Damascus as the terms of the mission are thrashed out later this week.
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 Himmiche's advance party.
"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."
Assad says Syria is under attack by foreign-backed terrorists 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.
Syria's official news agency SANA reported that four law enforcement members and a civilian were killed on Tuesday when "an armed terrorist group threw a bomb at a bus" in Aleppo, Syria's second largest city after the capital, Damascus.
It said terrorists were attacking and killing loyalist troops in their homes and kidnapping judges.