Syrian rebels sought to seal off President Bashar al-Assad's embattled regime from the outside world by seizing border posts amid calls for protests nationwide Friday on "the Ramadan of victory."

Members of the rebel Free Syrian Army (FSA) fought a raging battle with Syrian troops at the Bab al-Hawa border post with Turkey, an AFP photographer at the scene reported, addding that some 150 rebel fighters now controlled the crossing.

More than 300 people were killed across Syria on Thursday's deadliest day of the uprising now in its 17th month, as both China and Russia again dismayed the West by vetoing UN Security Council action against his regime for the third time.

Two days after rebels killed three of Assad's security chiefs including his brother-in-law, battles raged in the capital on Friday as the army said it had retaken the district of Midan from "terrorists," the term it uses for the rebels.

"Our brave army forces have completely cleaned the area of Midan in Damascus of the remaining mercenary terrorists and have reestablished security," state television said.

In Brussels, the European Union is preparing to freeze the assets of at least 26 Syrians close Assad and readying plans to board vessels and aircraft suspected of supplying weapons to his regime.

Syria on Friday began the first day of the Muslim holy fasting month of Ramadan amid a security forces ultimatum to "cleanse Damascus of terrorists."

Activists have call for fresh anti-regime demonstrations after the regular weekly prayers under the slogan "The Ramadan of victory will be written in Damascus."

The army on Thursday resorted to heavy armour in the capital for the first time, using tanks against the rebels.

Assad also appeared in public for the first time since Wednesday's bombing, greeting his new defence minister on state television as he scrambled to shore up his battered prestige. His previous defence minister was killed by the bomb.

Violence is spiralling across the country, according to the Syrian Observatory for Human Rights, which reported 302 people -- 98 Syrian soldiers, 139 civilians and 65 rebels -- killed Thursday nationwide.

At the Bab al-Hawa frontier crossing with Turkey, rebels sacked the Syrian border post, which was bloodstained and riddled with bullets.

They also looted Turkish lorries caught up in the battle.

Syrian soldiers had abandoned the site in the northwest province of Idlib, the scene of fierce fighting for months.

On Thursday, Iraq's deputy interior minister said the Free Syrian Army (FSA) had seized control of all crossings along their common border.

"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 seen FSA fighters seize a border outpost, detain a Syrian lieutenant colonel, and 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.

Meanwhile at the United Nations, Russia and China used their powers as permanent Security Council members 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.

Washington 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 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 on Friday, just hours before the end of the mission's 90-day mandate.

Russia said on Friday it backs an unconditional 45-day extension of the observer mission.

The upsurge in fighting has sparked a new exodus of refugees.

Nearly 19,000 Syrians poured 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 military had given residents 48 hours to leave areas of the capital, as security forces fought rebels pushing their "Damascus Volcano" offensive.

The authorities said state funerals would be held in Damascus on Friday for the three regime stalwarts killed in Wednesday's bombing.

Assad's brother-in-law and one of the Syrian security apparatus' hawks, Assef Shawkat, will be buried later 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.