Tanks raided a rebel bastion near Damascus on Sunday, activists said as UN observers toured Syrian hot spots, laying the ground for a 300-strong mission newly approved by the Security Council.
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 rebel fighters 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 violence comes only hours after the UN Security Council voted to approve Annan's plan to send the extra 300 unarmed observers to Syria for three months, although Washington warned it may veto a new mandate for the mission.
Tank shelling and heavy gunfire were reported as the early morning raid was launched to crush Assad's opponents in Douma, an outlying suburb of the Syrian capital, the 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 billowing into the sky, as gunfire and calls of "Allahu Akbar" (God is greatest) were heard in the background.
"Regime forces enter Douma each day, but today's assault was the largest," council member Mohammed Saeed told AFP.
The Syrian Observatory for Human Rights said two civilians were killed in Douma, while checkpoint guards shot dead a third overnight elsewhere in Damascus province.
Elsewhere, soldiers shot dead three civilians at a village in Jabal al-Zawiya district, northwestern Idlib province.
In Banias, an overnight ambush on a patrol killed one security forces member and wounded three others, the first such incident in the northern coastal city for nearly a year.
Two advance team members set up base in Homs on Sunday, a mission spokesman said, a day after they made their first visit to the central protest city since being deployed in Syria a week ago.
"Yesterday... 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," Neeraj Singh told AFP.
On Saturday, their visit to Homs included a stop in Baba Amr, a rebel hideout battered by a month-long army bombardment that killed hundreds, according to monitors, before it was retaken on March 1.
A YouTube video showed them meeting with 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.
"Because of that, we want you to stay. Please stay. This is what we want. When you come, shelling stops. When you come, killing stops," he told the observers, who wore blue helmets and bullet-proof vests marked "UN".
Only days after being deployed, the head of the advance team, Colonel Ahmed Himmiche of Morocco, acknowledged they faced a tough task to firm up the ceasefire Assad agreed to last week.
A spike in violence had already forced the Arab League to end its own Syrian monitoring mission in late January, barely a month after it was launched.
Under UN Resolution 2043, adopted on Saturday, 300 military observers will be sent to Syria for an initial period of 90 days if UN chief Ban Ki-moon determines it is safe to go.
The UN says well over 9,000 Syrians have been killed since democracy protests erupted in March 2011, inspired by uprisings that toppled autocrats in Tunisia, Egypt and an armed revolt that ousted and killed Libya's Moamer Gaddafi.
Monitors put the figure at more than 11,000, including at least 200 people killed in sporadic violence which has persisted since the UN-backed ceasefire took effect on April 12.
Russia's UN envoy Vitaly Churkin told the Security Council the resolution on the expanded mission was of "fundamental importance to push forward the process of the peaceful settlement in Syria."
But Washington warned it may prevent the mission's renewal after three months, while urging greater international pressure on Assad.
US ambassador Susan Rice told AFP Ban must make a "careful judgment" about conditions in Syria before sending the larger contingent of monitors.
"Our patience is exhausted. No one should assume that the United States will agree to renew this mission after 90 days," Rice told the Council.
The opposition Syrian National Council and rebel Free Syrian Army hailed the Security Council vote, saying it responds to the Syrian people's demands.