A United Nations advance observers' team arrived in the Syrian capital of Damascus late Sunday to monitor the fragile cease-fire brokered by international envoy Kofi Annan, sources told Xinhua anonymously.

The arrival of the six-member advance team came a day after the UN Security Council unanimously approved the observers' mission. The team will be backed with other batches of observers over the next period, and the total number of observers may eventually reach 250.

UN Secretary-General Ban Ki-moon welcomed the adoption of the resolution, which is the first legally-binding UN document since the outbreak of the crisis in the Middle East country in March 2011.

Russia and China, which had vetoed two previous UN resolutions forcing Syrian President Bashar al-Assad to step down, called on all parties to support Annan's mediation and cooperate with the advance team.

Vitaly Churkin, Russia's permanent representative to the UN, called on all parties to "establish favorable conditions including from the standpoint of ensuring security" for the UN observers.

"It is essential that all Syrian parties including the armed opposition quickly refrain from violence, strictly observe the Annan plan, and begin to organize a broad-based negotiating process," Churkin said.

Meanwhile, Chinese UN Ambassador Li Baodong said Saturday that "We need to guard against any attempt to create difficulties or trouble for Mr Annan's mediation."

The observers' mission came as part of the six-point plan by Annan that meant to solve the Syrian crisis politically.

The plan calls for a troop withdrawal from populated areas by April 10 and a cease-fire by April 12, and also for allowing in foreign media and peaceful demonstrations.

Syria has officially agreed on the plan and observed a cease- fire that went into effect last Thursday.

Meanwhile, Syrian presidential media advisor Buthiana Shabaan said Sunday that it is for Syria's interest to have international observers on its territories as they will help in exposing the crimes committed by armed groups.

In general, violence and death toll have to some extent declined in comparison with the previous stage, but clashes and attacks were still being reported during the fragile four-day-old truce.

At least six people, including a law-enforcement member, were killed Sunday across Syria in a new wave of violence, Syria's state-run SANA news agency reported.

While on the opposition side, the Britain-based Syrian Observatory for Human Rights said 10 civilians were killed Sunday, three of them in the central restive province of Homs. The Local Coordination Committees, an activists' network, put the civilian death toll on Sunday at 23.

Both opposition-biased organizations reported heavy shelling by government troops on al-Khalidiyah neighborhood in Homs, which is known to be a stronghold of rebel forces. Yet their account could not be independently verified.

The Syrian government has said the 13-month-old unrest in Syria is the work of armed groups and Islamic fundamentalists backed by foreign conspiracy.

On Sunday, Syria's Ministry of Defense said "armed terrorist groups" have hysterically ramped up their assaults on government troops since the army observed the UN-backed cease-fire, and stressed that it will prevent those armed groups from carrying on with criminal acts.

The ministry stressed that, out of the sense of responsibility toward protecting the lives of the citizens and preserving the country's security, it will stop "those terrorists from practicing more murders and sabotage acts."

The UN has estimated that more than 9,000 people have so far been killed in the Syrian conflict, while Damascus says 6,044 have died, including 2,566 soldiers and policemen.