The United Nations Security Council has unanimously voted to authorise the deployment of unarmed observers to Syria to monitor the country's ceasefire.

It is the council's first ever resolution on the Syrian crisis, which began more than 13 months ago.

The resolution approved sending the first 30 unarmed military monitors, who left for Syria immediately, arriving on Sunday (local time).

It also calls on the Syrian government to visibly implement all aspects of a UN-backed peace plan.

Following new attacks by government forces, which killed at least six people, the United States and its allies in turn raised "serious doubts" that president Bashar al-Assad will keep to a fragile ceasefire in place since Thursday.

The first five or six military monitors took planes from New York as soon as the UN Security Council passed the resolution, UN peacekeeping spokesman Kieran Dwyer said.

Others will follow in coming days, he added.

"They will quickly need to reach out to contacts both within the Syrian government, and their security forces, and with opposition forces so that both sides fully understand the monitoring role," Mr Dwyer said.

The resolution also calls upon the Syrian government to "implement visibly" all commitments under special envoy Kofi Annan's peace plan - including the withdrawal of all troops and heavy guns from Syrian cities.

The 15-nation council said it would assess the implementation of the resolution and "consider further steps as appropriate".

The leading powers had hoped for a vote on Friday, but this was held up by Russian objections to much of the text.

Russia's ambassador Vitaly Churkin said "substantive changes" had been made to make it "more balanced". He said there had been "sometimes heated" talks on the resolution.

Russia and China blocked the two previous resolutions saying they were aimed only at regime change.

A new resolution will be required to give a mandate to the full monitoring mission of more than 200 observers.

The resolution called on president Bashar al-Assad and the opposition "to guarantee the safety of the advance team" and its "freedom of movement and access".

The Security Council ordered UN secretary-general Ban Ki-moon to report by Thursday on the initial work of the monitors, and by Wednesday give recommendations for deploying the full mission.

US ambassador Susan Rice said new attacks on Saturday by Syrian security forces, including the shelling of rebel districts in Homs and firing on funeral mourners in Aleppo, raised "serious doubts" about Mr Assad's commitment to the ceasefire.

Other western envoys raised similar doubts.

Meanwhile, Mr Ban says at least a million people have been displaced inside Syria during the government's 13-month crackdown on dissent.

"We are very much concerned that at least one million people have been displaced inside Syria and there are still many Syrian refugees in neighbouring countries," he said after his meeting with Kofi Annan in Geneva.

Mr Ban says UN humanitarian chief Valerie Amos will host a meeting in the Swiss city next week to begin the task of getting aid to Syria's needy.

"I hope that [it] will be able to mobilise the necessary humanitarian resources so that all needy people will be able to get the necessary humanitarian assistance," he said.

ABC/AFP