DAMASCUS - Rebels seized control of Syria's border crossings with Iraq on the bloodiest day of the uprising against 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 on Thursday came as the army focused 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.
Assad 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.
Meanwhile at the UN in New York, Russia and China used their powers as permanent members of the UN Security Council to block resolutions on Syria for the third time in nine months.
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.
The US condemned the "highly regrettable decision" of China and Russia to veto the UN resolution, with President Barack Obama's spokesman Jay Carney warning of "repercussions... in terms of how they're viewed by the Syrian people".
"There's no doubt that Syria's future will not include Bashar al-Assad. His days in power are numbered," he warned.
Washington 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 also 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.
Britain and Pakistan proposed rival resolutions extending the UN Supervision Mission in Syria (UNSMIS) and, amid Council deadlock, a vote on both could be held Friday, just a few hours before the end of the mission's 90-day mandate.
The United Nations swiftly sent its top military official to Syria to take charge of the observer mission as the Security Council wrangled over the future of the operation.
Russia accused Western governments of seeking a pretext for military intervention.
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 considering 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.
At least 248 people, including 109 civilians, died in fighting Thursday, the highest toll since the turmoil began, the Syrian Observatory for Human Rights said.
The daily toll also included 93 government troops, and 46 militants and deserters, according to the British-based observatory's head, Rami Abdel Rahman.
The Syrian observatory said rebels had also seized control of a post on the border with Turkey.
The upsurge in fighting sent a new exodus of refugees fleeing across Syria's borders.
Nearly 19,000 Syrians pouring into Lebanon, a security official told AFP in Beirut, while Iraqi officials said thousands of Iraqi refugee families had fled home from Syria.
The Syrian army gave residents 48 hours to leave areas of the capital, where clashes were 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 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," Rahman added.
The authorities said state funerals would be held in Damascus on Friday for the three senior regime officials killed in Wednesday's bombing. The bodies would then be returned to their home towns for burial.
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".