Nawaf al-Fares, who has close ties to the Syrian security services, was the first senior diplomat to desert Assad, following hard on the heels of Manaf Tlas, a brigadier general in the elite Republican Guard and a close friend of Assad.
Assad's swift and bloody crackdown on what began as a broad, peaceful pro-democracy movement helped turn it into an armed rebellion. But the insurgents cannot match the army's firepower, and instead need to erode the loyalty and conviction within Assad's establishment to loosen its hold on power.
Tlas, the son of a veteran former Syrian defence minister, has made no public comment since fleeing to Paris. But Fares posted 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 ...
"Every Syrian man has to join the revolution to remove this nightmare and this gang," he said, accusing the Assad family and its allies of corruption and "destroying society" for 40 years.
The defection was seized on by Assad's opponents, but also by Western and Sunni Arab powers who insist, like the opposition, that Assad must leave power in any political settlement for Syria.
In Damascus, a terse government statement said: "The Syrian foreign ministry declares that Nawaf al-Fares has been relieved of his duties and he no longer has any link to our embassy in Baghdad or the foreign ministry. They embassy in Iraq will continue carrying out its normal duties."
With events on the ground outrunning diplomatic efforts, Britain on Wednesday circulated a draft resolution, backed by the United States, France and Germany, to make compliance with a transition plan drafted by international envoy Kofi Annan enforceable under Chapter 7 of the U.N. Charter.
This would allow the council to authorise actions ranging from diplomatic and economic sanctions to military intervention.
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 ceasefire nor political dialogue since it was agreed in April.
The draft in particular 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.
But Assad's chief backer on the U.N. Security Council, Russia, remained firmly in the Syrian leader's camp, having submitted its own draft resolution on Tuesday that made no mention of sanctions.
Russia's Deputy U.N. Ambassador Alexander Pankin told reporters after Annan briefed the council in New York on Wednesday that Moscow believed sanctions were a "last resort".
Russia and China, both veto-wielding permanent council members, have for months blocked attempts to increase the pressure on Assad, endorsing his argument that he is defending his country against armed groups, backed by the West and by Sunni Arab Gulf monarchies, bent on toppling the government.
Most of Assad's political and military establishment are members of the minority Alawite sect, an offshoot of Shi'ite Islam.
However, both Tlas and Fares are Sunnis, and their defections may indicate a growing alienation among the Sunni business elite, which had been slow to embrace a revolt that began among poorer parts of the majority community.
"Although Ambassador Fares is not a member of Assad's inner circle, he's a respected Sunni figure, and such a courageous act could help sway other Sunni elites to follow in his footsteps," said one U.S. official, speaking on condition of anonymity.
Western supporters of the rebel cause have been active in encouraging defections, which they see as one of the few tools available to them at the moment to undermine Assad.
Just the beginning?
Fares, who had held senior positions under the president's late father and predecessor Hafez al-Assad, is from Deir al-Zor, an eastern city on the road to Iraq that has seen a ferocious onslaught by the armed forces over the past few months.
"This is just the beginning of a series of defections on the diplomatic level. We are in touch with several ambassadors," said Mohamed Sermini, a member of the main opposition umbrella group, the Syrian National Council.
Assad's opponents say just under 13,000 armed and unarmed opponents of Assad, and around 4,300 members of security forces loyal to Damascus, have been killed since he launched a crackdown 16 months ago, using tanks and helicopter gunships to attack rebel strongholds inside Syria's biggest cities.
Activists on Wednesday reported a new bombardment of rebel areas of Homs, a bastion of opposition to Assad, as well as fighting in many other parts of the country. State media reported on missile tests, part of war games that analysts say are a warning to Assad's foes.
In Moscow, Syrian opposition talks with Russia ended in discord on Wednesday, and an opposition leader accused Moscow of pursuing policies that were helping to prolong the bloodshed in the country..
"The Syrian people don't understand Russia's position. How can Russia keep supplying arms? How can they keep vetoing resolutions? There needs to be an end to mass killings," said Burhan Ghalioun of the exile Syrian National Council (SNC).
But one member of the SNC delegation said there were signs that Russia, which has stepped up its diplomacy in recent weeks, may be moderating its support for Assad as turmoil engulfs its long-time ally.
"We're trying to work out what exactly Russia is trying to do here. I think they're looking for a genuine solution," said the SNC delegate, asking not to be identified.