Rebels seized control of all of Syria's border crossings with Iraq on Thursday, dealing a new blow to President Bashar al-Assad, as China and Russia dismayed the West by blocking UN action against his regime.
The rebel offensive on Syria's eastern border came as the army focussed its resources on Damascus, resorting to tank fire in the capital for the first time in its efforts to root out rebel fighters a day after a bomb blast killed three of Assad's top aides.
The president appeared in public for the first time since the bombing, greeting his new defence minister on state television, as he scrambled to shore up his battered prestige.
It was the third time in nine months that Russia and China had used their powers as permanent members of the UN Security Council to block resolutions on Syria.
UN-Arab League envoy Kofi Annan, who had called on the council to impose "consequences" for the failure to carry out his peace plan for Syria, expressed disappointment that it had failed to reach agreement.
Washington said the vetos meant the 300-strong UN observer mission deployed in Syria to oversee Annan's peace blueprint would now have to be withdrawn when its current mandate expires on Friday.
Iraq's deputy interior minister said fighters of the rebel Free Syrian Army had seized control of all crossings along the two countries' border, adding that Baghdad was mulling closing the frontier following bloody rebel reprisals against regime troops.
"All the border points between Iraq and Syria are under the control of the Free Syrian Army," Adnan al-Assadi told AFP by telephone.
Assadi said Iraqi border guards had witnessed the Free Syrian Army take control of a border outpost, detain a Syrian army lieutenant colonel, and then cut off his arms and legs.
"Then they executed 22 Syrian soldiers in front of the eyes of Iraqi soldiers."
The account of the killings could not be independently verified.
Assadi added: "If this situation continues, we are going to close the entire border with Syria."
The Syrian Observatory for Human Rights said rebels had also seized control of a post on the border with Turkey.
"Rebel fighters seized control of the Bab al-Hawa crossing (in the northwestern province of Idlib)," the Britain-based rights group said, adding that the rebels removed a photograph of Assad that was displayed there.
The upsurge in fighting, which claimed some 200 lives on Thursday alone according to the Observatory's figures, sent a new exodus of refugees fleeing across Syria's borders.
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Western governments expressed outrage at the decision by Beijing and Moscow to again block UN action against Assad's regime.
"The United Kingdom is appalled at the veto of Russia and China," said British Ambassador Mark Lyall Grant, whose country took the lead in drafting the resolution.
The text, backed by the United States, France, Germany and Portugal, called for non-military sanctions under Chapter VII of the UN Charter if Assad did not withdraw heavy weapons from Syrian cities in 10 days.
Russia accused Western governments of seeking a pretext for military intervention.
The resolution sought to "open the path to the pressure of sanctions and further to external military involvement in Syrian domestic affairs," said Russian Ambassador Vitaly Churkin.
The United States said the Security Council had "utterly failed" on Syria and that it would now work outside of the council to confront Assad's regime.
The White House said that without the tougher mandate the vetoed text would have implied, there was no point in retaining UN military observers in Syria to monitor the non-implementation of Annan's plan by Assad's government.
"The United States doesn't support sending unarmed UN employees into Syria to try to observe the brutality of the Assad regime when there's no mechanism within the resolution to create consequences for the regime," said White House spokesman Jay Carney.
The Syrian army gave residents 48 hours to leave areas of the capital, where clashes are taking place between security forces and rebels pushing their "Damascus Volcano" offensive.
"These extremely violent clashes should continue in the next 48 hours to cleanse Damascus of terrorists by the time Ramadan begins" on Friday, a security source told AFP, referring to the Muslim fasting month.
The Syrian Observatory for Human Rights said that in the western district of Mazzeh alone, hundreds of people were on the move, "fearing a large-scale operation by regime troops."
"The army stormed the Qaboon district with a large number of tanks," the Observatory's Rami Abdel Rahman added.
The authorities announced that state funerals will held on Friday for the three senior regime officials killed in Wednesday's bombing.
"State funerals will be held in Damascus tomorrow, and then each of the bodies will be transported to his native town to be buried there," a security source told AFP.
Assad's brother-in-law and one of the Syrian security apparatus' hawks, Assef Shawkat, will be buried in the western province of Tartus.
Daoud Rajha, who was defence minister, will be buried in his Christian town of Maalula near Damascus, and crisis cell chief Hassan Turkmani in northern Aleppo.
The leader of the opposition Syrian National Council, Abdel Basset Sayda, said the deaths of the top Assad aides showed "the regime is in its final days."