The first ambassador to abandon Syrian President Bashar al-Assad called on the army to "turn your guns on the criminals" of the government, as troops backed by tanks swarmed into a suburb of Damascus on Thursday to flush out rebels.
Nawaf al-Fares, who has close ties to the security services, was Syria's ambassador to its neighbour Iraq, one of its few friends in the region. Coming just days after the desertion of Manaf Tlas, a brigadier general in the elite Republican Guard who grew up with the president, his defection gave the anti-Assad uprising one of its biggest boosts in 16 months of bloodshed.
But Syria's strongest ally, Russia, stuck by Assad on Thursday with a clear warning to his Western and Arab enemies that it would not even consider calls for a tough new resolution by the UN Security Council in New York.
Britain circulated a draft on Wednesday, backed by the United States, France and Germany, that would make compliance with a transition plan drafted by international envoy Kofi Annan enforceable under Chapter 7 of the UN Charter.
This would allow the council to authorise actions ranging from diplomatic and economic sanctions to military intervention.
But as Council members began negotiations on a resolution to renew the UN Syria monitoring mission, Russia's Deputy UN Ambassador Alexander Pankin made clear Moscow would use its veto if it had to.
"We are definitely against Chapter 7," he told reporters. "Anything can be negotiated, but we do not negotiate this, this is a red line."
Annan himself asked the 15-member council to agree on "clear consequences" if the Syrian government or opposition failed to comply with his plan, which has produced neither a cease-fire nor political dialogue since it was agreed in April.
The British draft threatens the Syrian government with sanctions if it does not stop using heavy weapons and withdraw its troops from towns and cities within 10 days.
While the insurgents cannot match the Syrian army's firepower, they have managed to establish footholds in towns, cities and villages across Syria, often prompting Assad's forces to respond with helicopter gunships and artillery.
On Thursday, residents reported the first bombardment of the capital as security forces used mortars, then tanks and infantry to try to flush rebels near Kfar Souseh, a southern suburb.
The al-Lawan and Basateen districts of Kfar Souseh are a semi-residential area of olive and fruit groves lying astride an interchange on the capital's Hafez al-Assad southern bypass.
Activists said tanks had fired from the Hadi Mosque to the east and al-Mazzeh military airport immediately to the west. "I woke up this morning and saw helicopters flying over the area. Then I started hearing the mortars. There were about six or seven of them in the past half hour," said anti-government activist Hazem al-Aqad. "People are terrified, families are getting in their cars and rushing away as fast as they can."
The official news agency SANA said Syrian forces killed rebels shipping arms in two boats on Lake Qotaina, near Homs.
Assad's opponents say 13,000 armed and unarmed opponents of Assad, and 4,300 members of security forces loyal to Damascus, have been killed since the uprising began 16 months ago. The activist Syrian Observatory for Human Rights said 42 people had been killed so far on Thursday, 30 of them civilians.
Brigadier Tlas, socialite son of a veteran former defence minister, has made no public comment since fleeing, but France said on Thursday he was in contact with Syrian rebels.
"I know the opposition and this general have approached each other contact has been made," said Foreign Minister Laurent Fabius. But he said he could not confirm Tlas was in Paris.
Fares by contrast immediately went public, posting a video statement on Facebook on Wednesday that repeatedly said government forces had been killing civilians.
"I declare that I have joined, from this moment, the ranks of the revolution of the Syrian people," he said.
"I ask ... the members of the military to join the revolution and to defend the country and the citizens. Turn your guns on the criminals from this regime ...
Iraqi Foreign Minister Hoshiyar Zebari said Fares was now in Qatar, one of the Gulf states overtly supporting the rebels and calling for Assad to go.