Syrian forces reportedly killed five civilians in shelling of rebel areas and clashes with gunmen on Sunday, testing a shaky UN-backed ceasefire as international monitors prepared to fly in.

UN chief Ban Ki-moon said he was "very much concerned" at the renewed killings in the flashpoint central city of Homs and urged the Damascus government to ensure that the ceasefire does not collapse.

State-run news agency SANA meanwhile reported that Foreign Minister Walid Muallem will visit ally China at an unspecified date to discuss the mission of UN-Arab League envoy Kofi Annan who designed the ceasefire.

Forces loyal to President Bashar al-Assad subjected the Khaldiyeh and Bayada neighbourhoods of Homs to their fiercest bombardment since the truce came into force at dawn on Thursday, monitors said.

"The bombardment of Khaldiyeh intensified this morning with an average of three shells a minute," the head of the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.

Three civilians were killed in Homs city, including one in Khaldiyeh, he said.

With clashes warming up and both sides blaming each other for the violence, shabiha pro-regime militiamen also shot dead a civilian in the central province of Hama and another man was killed by security forces in the Damascus suburb of Douma, the Observatory said.

Three civilians died in Homs shelling on Saturday, among 14 people reported killed nationwide ahead of a UN Security Council vote approving the dispatch of the observer mission to monitor the truce.

Thirty-two people have been killed since the ceasefire brokered by Annan took effect, most of them civilians, the Observatory said.

The death toll is sharply down on pre-ceasefire levels after Syria announced it was halting military operations against the rebels on Thursday.

But regime forces have so far ignored another key element of Annan's peace plan -- an undertaking to withdraw tanks from towns and cities, the Observatory said.

The authorities on Sunday charged that rebels had "intensified" attacks on security forces and civilians, warning of a response, as state media published a list of alleged acts of violence.

Security forces "will prevent the terrorist groups from continuing their criminal attacks," said a military official quoted on state media, accusing the rebels of a deliberate escalation to wreck the truce.

Ban voiced concern over the shelling of Homs.

"I am very much concerned about what has happened since yesterday and today," he said. "It is important, absolutely important, that the Syrian government should take all the measures to keep this cessation of violence."

The UN secretary general said he would present on Thursday his proposal to enlarge the UN monitoring mission, which will have 30 unarmed military observers at first, to 250 people.

The first half-dozen observers from an advance team for the mission were due in the Syrian capital on Sunday, although their flight plans were unclear.

The first group boarded a plane from New York straight after the Security Council resolution was passed.

The next 25 will come from missions around the Middle East and Africa "so we can move people quickly and they are experienced in the region," UN peacekeeping department spokesman Kieran Dwyer told AFP.

China and Russia, which raised earlier reservations over the Western-drafted text and vetoed past resolutions, backed the vote, ensuring passage of the first Security Council resolution on Syria since the uprising erupted in March 2011.

But Moscow and Beijing also said they are putting increased pressure on Damascus.

State news agency SANA said Muallem would go to China on a two-day visit at the invitation of his Chinese counterpart Yang Jiechi.

"During the visit, Muallem will discuss means of enhancing bilateral relations and the mission of... Annan," the agency reported giving no date for the visit.

UN Resolution 2042 approved the sending of 30 unarmed military observers as soon as possible and called on both Syrian government and opposition forces to halt "armed violence in all its forms."

It also urged the government to "implement visibly" all its commitments under Annan's peace plan, including the withdrawal of all troops and heavy guns from cities.

Assad and the opposition must also "guarantee the safety of the advance team without prejudice to its freedom of movement and access," and the "primary responsibility" for observers' safety will rest with the Syrian government.

The resolution's passage was welcomed by Syria's main opposition.

"We are ready to act to make the Annan plan a success," the Syrian National Council said in a statement signed by its leader Burhan Ghalioun.

EU foreign policy chief Catherine Ashton said the resolution "presents an unequivocal call from the international community to the Syrian regime to stop violence against its population and to address urgent humanitarian needs."

The opposition has repeatedly demanded the sending of international observers, and called on Syrians to take advantage of the ceasefire to stage demonstrations.

The United Nations says more than 9,000 people have been killed since the uprising began. Monitors say the death toll has topped 10,000.

burs/hkb