Opposition activists reported Monday intense shelling of rebel strongholds in central Homs province for the third consecutive day as an advance team of UN observers arrived in Syria a day earlier to monitor the UN- brokered cease-fire.

The Britain-based Syrian Observatory for Human Rights said that government troops are trying to take control of neighborhoods in Homs, pounding them with mortar fire. It said the shelling targeted the districts of Khalidyah and Bayyada.

The activists account couldn't be independently verified.

Homs, Syria's third-largest city and home to 1 million citizens, has emerged as a main stronghold for armed rebels.

On Thursday, Syria observed a cease-fire truce brokered by international envoy to Syria Kofi Annan.

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 five-day-old truce.

Meanwhile, an advance team of UN observers arrived in Syria Sunday night to prepare for the advent of more observers to monitor the cease-fire on ground.

Head of the UN advance team of observers to Syria said Monday that his five-member team is bracing to begin their task as soon as possible.

"We are going to organize ourselves in order to do our task as soon as possible... I'm very optimistic... all peace keepers are optimistic," Moroccan Colonel Ahmed Himmiche told reporters while leaving his hotel along with other observers Monday.

The team's arrival 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.

Syria's presidential media advisor Buthiana Shabaan said Sunday that sending international observers to Syria would help in exposing the parties that are carrying on with murder, destruction and kidnapping, stressing that it is for Syria's own interest to have observers on its territories.

After the cease-fire truce went into effect on Thursday, the armed groups have carried out more than 60 attacks in several Syrian cities, targeting civilians, army and security personnel, Buthiana Shabaan told BBC Sunday.

She noted that these violations have been documented and sent to the UN envoy to Syria Kofi Annan.

The Syrian government blames the year-long crisis in Syria on armed groups working out a foreign conspiracy.

The UN estimated that more than 9,000 people have so far been killed in the Syrian conflict, including at least 500 children, while Syria says 6,044 have died, including 2,566 soldiers and police.