Troops shot dead six civilians on Sunday in Homs despite the presence of UN military observers in the rebel province to pave the way for a 300-strong mission approved by the Security Council, monitors said.
The deaths raised to 17 the number of people killed the same day across Syria, including two civilians and four soldiers who died during an assault on a rebel bastion near Damascus by regime forces, they said.
The violence came even as UN-Arab League envoy Kofi Annan singled out the government of President Bashar al-Assad in an appeal for an end to hostilities by both loyalist forces and rebels seeking to oust him.
"I urge all forces whether governmental, opposition or others to put down their weapons and work with the United Nations monitors to consolidate the fragile cessation of violence," Annan said in a statement.
"The government in particular must desist from the use of heavy weapons and, as it has committed, withdraw such weapons and armed units from population centres and implement fully its commitments under the six-point plan."
The latest bloodshed came only hours after the Security Council voted to send 300 unarmed observers to Syria for three months, although Washington warned it may veto a new mandate for the mission.
The Syrian Observatory for Human Rights said regime forces shot dead three civilians in the city of Homs and three others in Talbisseh, another town visited by UN observers in Homs province.
"Three citizens were killed by security forces fire in the Khaldiyeh, Al-Guta and Al-Mukayam districts of Homs," the Britain-based monitoring group said.
Two advance team members set up base in Homs on Sunday, mission member Neeraj Singh said earlier, a day after they made their first visit to the central protest city since arriving in Syria a week ago.
On Saturday "the team drove or walked around the city of Homs and stopped at different locations to talk to the people. Two UN military observers have now been stationed at Homs since yesterday evening," Singh told AFP.
UN observers also toured on Sunday the rebel city of Rastan in Homs province, according to Free Syrian Army spokesman Colonel Saadeddine Kassem who escorted them.
Troops fired warning shots into the air to disperse a group of residents who surrounded the observers, he added.
Later the observers visited the city of Hama farther north, including a square which witnessed huge anti-regime protests last year, activist Abu Ghazi Hamwi reported from the scene.
The official Syrian news agency SANA said the observers met the governor of Hama during the visit.
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On Saturday, the observers went to Baba Amr, a rebel hideout in Homs battered by a month-long army bombardment that monitors say killed hundreds before it was retaken on March 1.
A YouTube video showed the observers meeting activists who begged them to stay. Its authenticity could not be verified.
"Today is the first day since two months, exactly since 5 February... in Homs without shelling... without killing, without fire," one unidentified activist said in the footage.
In other violence on Sunday, tank shelling and heavy gunfire were reported in Douma, an outlying rebel suburb of Damascus, activists reported.
"Regime forces backed by tanks stormed Douma under heavy gunfire," said the revolutionary council of Damascus province.
Videos posted online showed towering columns of smoke over Douma, as gunfire crackled and calls of "Allahu Akbar" (God is greatest) were heard.
Council member Mohammed Saeed told AFP the assault was "in retaliation for huge anti-regime protests there and because it's a centre of dissent."
The Observatory said four soldiers were killed when a bomb targeted an armoured personnel carrier in Douma, where two civilians were also shot dead.
A third civilian was shot dead at a checkpoint elsewhere in Damascus province.
In northwest Syria, it said, soldiers shot dead three civilians at a village in Jabal al-Zawiya district of Idlib province while an overnight ambush on a patrol in Banias killed one security officer and wounded three.
Under UN Resolution 2043, 300 military observers will be sent for an initial 90 days if UN chief Ban Ki-moon determines it is safe to go.
Finland announced Sunday it would contribute 10 soldiers to the force, although military officers voiced concern that the mission was ill-defined and risky.
"The funniest thing is this scarce information... we are going somewhere to do something. We have no other information. Let's just see what happens, but we're ready for anything," Finnish training officer Rolf Kullberg said.
Saturday's vote in New York was hailed by Syria's political and military opposition, but Washington warned it may prevent the mission's renewal after three months, while urging greater international pressure on Assad.
The UN says well over 9,000 Syrians have been killed since democracy protests erupted in March 2011, while monitors put the figure at more than 11,000.