Government troops pounded a Damascus suburb with artillery and heavy machine guns as United Nations ceasefire monitors toured a rebel-held town in central Syria with army defectors, activists said on Sunday.

The shelling in Douma highlighted the need for more observers a day after the Security Council voted to expand the number of UN observers from 30 to 300 in hopes of salvaging an international truce plan marred by continued fighting between the military and rebels.

An eight-member team is already on the ground in Syria, and since Thursday has visited flashpoints of the 13-month-long conflict. Fighting generally halts temporarily when the observers are present in an area, but there has been a steady stream of reports of violence from towns and regions where they have not yet gone.

"This UN observers thing is a big joke," said Douma-based activist Mohammed Saeed. "Shelling stops and tanks are hidden when they visit somewhere, and when they leave, shelling resumes."

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Taken from an amateur video released by the Shaam News Network, a Free Syrian Army solider accompanys UN observers in Rastan town in Homs, Syria. (Shaam News Network/AP)

His comments reflect a widespread lack of faith among many Syrians in international envoy Kofi Annan's cease-fire plan for ending the violence in Syria and launching talks between President Bashar al-Assad and those trying to oust him. Syria's opposition and its Western supporters suspect Assad is paying lip service to the truce since full compliance —including withdrawing troops and heavy weapons from populated areas and allowing peaceful demonstrations — could quickly sweep him from power.

A previous observer team, dispatched by the Arab League at the start of the year, withdrew after a month after failing to halt the fighting.

Syria's state-run news agency SANA said UN monitors visited the central city of Hama Sunday where they met with the governor, while opposition activists said observers visited Rastan, a rebel-held town south of Hama. An amateur video posted online showed two white UN vehicles driving in Rastan accompanied by a red pick up truck with the words "Free Army" written on it.

Other videos showed two UN monitors wearing blue helmets and body armour touring Rastan along with officers from the rebel Free Syrian Army who point to damaged buildings and a large crowd of people shouting "Bye Bye Bashar!" and "The people want to topple the regime."

Saeed, the activist, said two people were killed Sunday by indiscriminate firing in Douma, which was the scene of intense clashes between rebels and security forces before the U.N.-brokered cease-fire went into effect more than a week ago.

At least 12 people in total were killed in regime attacks Sunday, according to The Syrian Observatory for Human Rights including three people in the northern province of Idlib and one person in the village of Hteita outside Damascus when troops opened fire from a checkpoint.

Security forces killing people

The Observatory, a Britain-based opposition group with a network of activists on the ground, also said four soldiers were also killed when a roadside bomb hit an armoured personnel carrier in the town later Sunday.

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An image provided by the Local Coordination Committees shows Syrian security forces in the southern city of Daraa. (Local Coordination Committes/AP)

It was not immediately clear what prompted the attack on Douma. Saeed said loud explosions that shook the city early Sunday caused panic among residents, some of whom used mosque loudspeakers to urge people to take cover in basements and in lower floors of buildings.

The Security Council approved a resolution Saturday expanding the UN observer mission from 30 to 300 members, initially for 90 days. The expanded force is meant to shore up the ceasefire that officially took effect 10 days ago, but has failed to halt the violence that the UN says has killed more than 9,000 people since March 2011.

Annan on Sunday welcomed the vote, calling it a "pivotal moment" in the process of stabilizing the country and urged all Syrians to uphold the cease-fire.

"The government in particular must desist from the use of heavy weapons and ... withdraw such weapons and armed units from population centers," he said.

'Gross violations' of the rights of Syrian people

UN chief Ban Ki-moon has accused Assad of violating the truce, and said Saturday that "the gross violations of the fundamental rights of the Syrian people must stop at once." Rebel fighters have also kept up attacks.

The official SANA news agency said Sunday that an officer was killed and 42 others wounded in a roadside bomb explosion that targeted their bus Sunday in northern Syria. Two other explosives were dismantled on the spot on the Raqqa-Aleppo highway, SANA said.

In Cairo, a spokeswoman for the opposition Syrian National Council, Bassma Kodmani, called for the number of UN monitors in Syria to be raised to at least 3,000, saying the current 300 approved by the UN would not be enough.

The eight-member advance team has already visited the Damascus suburb of Arbeen, the southern province of Daraa, and the battered opposition stronghold of Homs. The monitors have not visited Douma yet.

Activists said Homs was relatively calm for the second consecutive day Sunday. But the Observatory said six people were killed in the province — three of them in gunfire during raids in search of fugitives in farms near the town of Talbiseh and three others in gunbattles in the Khaldiyeh and Ghouta districts of Homs.

Five monitors who toured Homs Saturday encountered unusually calm streets after weeks of shelling, and activists said it was the first quiet day in months. Two observers stayed behind in Homs to keep monitoring the city, after the rest of the team left that evening.

Activist Salim Qabani, based in Homs province, said Sunday there was none of the heavy shelling of the previous days.