UN observers monitoring the ceasefire in Syria yesterday visited several rebellious suburbs near the capital and were met by thousands of protesters demanding the collapse of the regime.

Meanwhile, Syrian forces shelled a district of the central city of Hama yesterday and opened fire on residents, killing at least 20 people, activists said.

The incident occurred in the Arbaeen neighbourhood a day after United Nations observers, monitoring an 11-day-old ceasefire agreement, visited Hama, a hotbed of a popular revolt against President Bashar al-Assad.

Amateur video posted by activists on YouTube showed four of the unarmed observers in blue helmets walking in Douma, a northern suburb of Damascus, surrounded by a huge crowd waving Syria independence flags.

"The people demand the fall of the regime," some chanted while others called for the arming of the rebel Free Syrian Army.

Monitors also visited the town of Zabadani, 47 kilometres northwest of the capital, where regime forces and rebels have clashed repeatedly in past months.

Fares Mohamed, an activist in Zabadani, said the observers' visit lasted barely half an hour.

"They refused to head to a location less than a kilometre from the town to see tanks hidden by the regime," said Mohamed, who was reached via Skype.

The observers are part of an eight-member advance team sent to Syria earlier this month to monitor a fragile ceasefire that went into effect April 12.

Two advance team members on Sunday set up base in the central city of Homs, scene of some of the fiercest fighting between government troops and rebels since the outbreak of the 13-month revolt against President Bashar al-Assad's regime.

The official state news agency SANA said the observers yesterday toured the battered city's Al-Waer neighbourhood.

A total of 30 observers are expected in Syria in the coming days pending the arrival of an expanded team of up to 300 observers approved by the UN Security Council as part of a truce brokered by international envoy Kofi Annan.

But it will be up to UN chief Ban Ki-moon to determine whether the situation is safe enough to deploy the 300 observers for an initial 90-day period.

Annan is to brief the Security Council on the situation today.

The United Nations estimates that more than 9,000 people have died in Syria since the uprising began in March 2011.

Activists have been sceptical of the UN observer mission, saying the regime was simply buying time and was not committed to the ceasefire plan.

Abu Omar, an activist in Damascus, said the observers were playing by the regime's rules by coordinating all their movements exclusively with the authorities.

"They are not coordinating with us and by not doing so, their mission does no good to the Syrian people," he told AFP. "Their mission is a failure because they are not working with people on the ground."