Two more senior Syrian diplomats have defected amid mounting pressure on the regime of President Bashar al-Assad, the US has confirmed.
Syria’s representatives in the United Arab Emirates and Cyprus – who are husband and wife – are reported to have fled to Qatar.
It comes amid intensifying clashes in the key city of Aleppo, where troops are trying to halt a rebel advance.
UN members have traded more accusations of blame amid the diplomatic impasse.
Earlier, UN Secretary General Ban Ki-moon urged the world to “act now to stop the slaughter”, but Security Council members remain deadlocked over what action it should take.
The Syrian government has rushed troops and tanks to Aleppo, Syria’s second city and commercial centre, parts of which were seized by rebels.
The heightening of the crisis is causing regional concern, say correspondents, amid a growing exodus of refugees and fears the fighting could draw in Syria’s neighbours.
Syrian diplomats’ defection comes amid intensifying clashes in the key city of Aleppo
“We can confirm the defections of Syrian ambassadors to both the UAE and Cyprus,” White House spokesman Jay Carney told reporters aboard Air Force One.
Jay Carney said the move showed that “senior officials around the Assad inner circle are fleeing the government because of the heinous actions taken by Assad against his own people, and the recognition that Bashar al-Assad’s days are numbered”.
The diplomats in question are Lamia al-Hariri, Syria’s charge d’affaires in Cyprus, and her husband Abdelatif al-Dabbagh, ambassador to the UAE.
A military attaché at the Syrian embassy in Oman – Mohammed Tahseen al-Faqir – is also reported to have defected.
Earlier this month, Nawaf Fares, Syria’s ambassador to Iraq, left for Qatar.
A senior state department official told AFP news agency: “These defections serve as a reminder that the bottom is starting to fall out of the regime. It is crumbling and losing its grip on power.”
Aleppo is now the focus of a battle which neither regime nor opposition forces can afford to lose.
He says restive neighbourhoods are being pounded by artillery, mortars and helicopter gunfire, and there are multiple reports of reinforcements heading to the city.
One activist based in the city, Mohammed Saeed, told Associated Press news agency they were expecting a big assault to try to reassert government control.
Adrien Jaulmes, of French newspaper Le Figaro, said that many people had fled Aleppo and others remained off the streets and in their homes.
“All afternoon, helicopters and Syrian jet fighters have been circling above the city, with the Free Syrian Army fighters firing at them with all the weapons they have,” he said, adding that the situation remained fluid and difficult to assess.
The fighting has caused renewed regional concern, with Turkey tightening its border controls with Syria, though it says it will allow refugees to get through.
Thousands of refugees have already sought shelter in Turkey, Lebanon, Jordan and Iraq.
Meanwhile, AP reported Israelis were rushing to get government-issue gas masks on Wednesday, following a Syrian threat on Monday that it would employ chemical weapons against external attackers.
On Wednesday, the UN’s Ban Ki-moon urged world leaders to halt the slaughter in Syria.
But further bitter accusations of blame followed within the UN Security Council, which has seen three resolutions blocked by Russia and China.
“The Syrian people will pay the price for this failure [to act],” Germany’s UN ambassador Peter Wittig told a Security Council debate on the Middle East on Wednesday.
But Russia envoy Vitaly Churkin retorted that pledges by some Western powers to take steps to support the Syrian opposition outside the council “contributes and leads to an escalation of confrontation”.
Major Western countries have decided to expel senior Syrian diplomats following the killing of 108 people in the Houla region of Syria on Friday.
France, the U.S., UK, Germany, Italy, Spain, Canada, Australia and the Netherlands all took action against Syria’s envoys.
Most of the victims in Houla were summarily executed, the UN says.
Residents said pro-government shabiha militia had entered homes and opened fire indiscriminately.
Syrian President Bashar al-Assad told UN-Arab League peace envoy Kofi Annan in Damascus that terrorists had stepped up their operations across Syria, including killing and kidnapping. His remarks were quoted by state TV.
UN human rights spokesman Rupert Colville said initial investigations had suggested that most of those killed in the village of Taldou, near Houla, were summarily executed.
Rupert Colville said 49 children and 34 women were among the victims. UN observers who visited Taldou said many of the victims had been killed by close-range gunfire or knife attacks.
Eyewitnesses said pro-government shabiha militiamen had carried out the killings. Survivors said they had hidden or played dead.
Syrian leaders insist that the massacre was the work of “terrorists”, aiming to derail the peace process and provoke intervention by Western powers.
Major Western countries have decided to expel senior Syrian diplomats following the killing of 108 people in the Houla region of Syria on Friday
US state department spokesperson Victoria Nuland announced that the Syrian charge d’affaires in Washington had been given 72 hours to leave the country.
“We hold the Syrian government responsible for this slaughter of innocent lives [in Houla],” Victoria Nuland said.
Canada denounced the Syrian government’s “heinous and murderous acts” while Australia described the Houla massacre as a “hideous and brutal crime”.
Spain talked of “unacceptable repression” and France’s new Foreign Minister, Laurent Fabius, described President Assad as “the murderer of his people”.
The French government said “the murderous folly” of the Damascus regime threatened regional security.
The Dutch declared Syria’s ambassador to the Netherlands, who is also ambassador to Belgium and lives in Brussels, as “persona non grata”.
Fellow EU state Bulgaria said it was expelling Syria’s interim ambassador and two other diplomats in protest at the killings in Houla.
Despite the show of protest, it is unclear whether the mass diplomatic expulsions will change much on the ground.
Syria’s charge d’affaires in London has been given seven days to leave.
“The international community is appalled by the violence that has continued, by the behavior of the regime, by the murder of so many innocent people,” British Foreign Secretary William Hague said.
President Assad said the success of Kofi Annan’s peace plan depended on halting what he called terrorist actions and stopping arms-smuggling.
Kofi Annan conveyed “the grave concern of the international community about the violence in Syria, including in particular the recent events in Houla”, the UN envoy’s office was quoted as saying by AFP news agency.
“He conveyed in frank terms his view to President Assad that the six-point plan cannot succeed without bold steps to stop the violence and release detainees, and stressed the importance of full implementation of the plan.”
Russia, which supplies arms to the Syrian government and has blocked UN resolutions calling for action against Damascus, has blamed both sides for Friday’s massacre.
Its Foreign Minister, Sergei Lavrov, expressed concern that “certain countries” were beginning to use the Houla massacre “as a pretext for voicing demands relating to the need for military measures to be taken”.
A meeting of the so-called Friends of Syria group is to meet in France in July, President Francois Hollande’s office said.