United Nations chief Ban Ki-moon urged "maximum restraint" from the Syrian regime and cooperation from the opposition Monday as UN military observers began to monitor a fragile ceasefire.

"It is very important that the cessation of violence must continue," he said at a news conference in Brussels. "The Syrian authoritiesmust exercise maximum restraint" and "the opposition forces should also fully cooperate."

Ban, who was in Brussels for a renewable energy conference, said the Syrian ceasefire accord remained "very fragile".

European Commission president Jose Manuel Barroso, speaking alongside the UN secretary-general, also said that "we call on the Syrian government to fully respect the six-point plan as a matter of urgency."

Ban said there were no immediate plans to provide military protection for the UN observers, who arrived in Damascus on Sunday.

"We're not contemplating military protection at this time because we think that freedom of movement should be guaranteed," he said.

Six members in a first advance party of observers have arrived and are to be joined by others, increasing the team to 30 monitors who were approved by the UN Security Council on Saturday.

Should the ceasefire hold, the team will be expanded to 200 people.

"They should be allowed to freely move," Ban added.

The first group will set up a headquarters and prepare routines so that the mission can verify that a cessation of hostilities, started Thursday, between President Bashar al-Assad's forces and opposition fighters is holding.

Syrian forces reportedly killed five civilians in shelling of rebel areas and clashes with gunmen in the hours before the arrival of the first monitors.