President seen for first time since blast that killed three top officials, while Russia and China veto censure of regime

Bashar al-Assad surfaced in Damascus to swear in a new defence minister to replace one of the three top officials whose killing stunned his regime as rebel fighters took over the country's border crossings into Turkey and Iraq and diplomacy reached a dead end.

Fighting was reported in the capital and across Syria as the president was seen for the first time since Wednesday's bomb blast in the heart of the city. It killed his brother-in-law and security chief, Assef Shawkat, as well as the defence minister and a third senior figure.

Rumours had circulated that Assad himself had been injured in the explosion or fled for the coastal city of Latakia with his wife, Asma, but state TV showed him swearing in General Fahad Jassim al-Freij as defence minister.

The capture of the border crossings – reportedly including all into Iraq – appeared to represent a dramatic new challenge to Assad's control and will likely prove crucial in funnelling arms and supplies into besieged rebel areas.

Hopes for any kind of peaceful resolution of the escalating crisis faded as Russia and China used their vetos at the UN security council to block a western-backed resolution calling for punishment if Assad failed to implement an internationally backed agreement on a ceasefire and political transition with the opposition.

The foreign secretary, William Hague, condemned the UN developments as "inexcusable and indefensible" and warned Assad that his regime was "doomed". The White House also condemned it as "highly regrettable."

An estimated 15,000-17,000 have been killed in the 16 months of bloodshed in Syria. Funerals were held in Damascus for Wednesday's bomb victims as Assad's sister Bushra and mother Anisa received condolences for the death of Shawkat.

Western diplomatic sources told the Guardian they believed the loss of the highly experienced Shawkat and Dawoud Rajha, the defence minister, would now make it harder for the regime to judge how much force it could use without risking alienating Russia, still Assad's most influential and supportive ally. In one of the most significant events on a fast-moving day, fighters of the Free Syrian Army took control of the border crossing into Turkey at Bab al Hawa – gateway to Idlib province – and Bab al Salam and Al-Boukamal crossings into Iraq, according to videos posted on the internet. That seems certain to make it easier for the FSA, being financed and armed by Saudi Arabia and Qatar, to step up their fight.

"Idlib province will be Syria's Benghazi," said one opposition activist – a reference to the way Libyan rebels were able to confront the Gaddafi regime last year once they had taken control of the country's second city.

Sana, Syria's official news agency, said that terrorists had been stopped while trying to cross the border from Lebanon.

The Syrian military, meanwhile, gave residents 48 hours to leave parts of Damascus held by rebel forces as it prepared a counter-attack aimed at seizing ground after four days of rebel gains. The FSA continued to hold areas it seized this week and is bracing for a decisive battle against loyalist forces.

In a reflection of mounting international tension over the crisis, Hague said he was appalled by the decision of Russia and China to veto the UN resolution. "What is happening in Syria is a tragedy for its entire people and a threat to international peace and security," he said. "The country is on the verge of total collapse as well as civil war." Kofi Annan, the UN and Arab League envoy to Syria, said he was "disappointed that at this critical stage the UN security council could not unite and take the strong and concerted action he had urged and hoped for."

Anti-Assad forces remain defiant. "The battle for, and in Damascus was inevitable, and will likely be the final chapter in the revolution," said one activist. "No one is sure how long or how bloody it will be. People are scared but resigned that its a necessity to rid the country of this regime once and for all. Very likely, the FSA will use urban warfare tactics, attacking then vanishing, not holding ground, but disrupting the government and capital."

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