Western powers vowed to seek new ways to punish Damascus amid growing outrage after Russia and China blocked a UN resolution condemning Syria for its deadly crackdown on protests.
The vetoes wielded by Beijing and Moscow at the UN Security Council on Saturday handed President Bashar al-Assad's regime a "licence to kill" according to the opposition.
The rare double veto also drew international condemnation, with US Secretary of State Hillary Clinton calling it a "travesty" and vowing to push for new sanctions on Syria.
"Those countries that refused to support the Arab League plan bear full responsibility for protecting the brutal regime in Damascus," a forceful Clinton told a news conference with Bulgarian Prime Minister Boyko Borisov in Sofia.
Faced with a "neutered Security Council" she promised to redouble efforts outside of the UN.
"We will work to seek regional and national sanctions against Syria and strengthen the ones we have," Clinton added.
Echoing Washington's sentiments, France said Europe said they would strengthen sanctions against Damascus.
"Europe will again harden sanctions imposed on the Syrian regime. We will try to increase this international pressure," French Foreign Minister Alain Juppe said.
He also said France would "help the Syrian opposition to structure and organise itself."
Russia defended its UN veto, saying Western powers had refused to reach a consensus.
"The authors of the draft Syria resolution, unfortunately, did not want to undertake an extra effort and come to a consensus," Deputy Foreign Minister Gennady Gatilov wrote on Twitter.
Russia's Foreign Minister Sergei Lavrov and Foreign Intelligence Service chief Mikhail Fradkov are preparing to visit Damascus on Tuesday, amid reports that the mission could try to push Assad to quit.
"Russia strongly intends to achieve a rapid stabilisation of the situation in Syria through the rapid implementation of much-needed democratic reforms," the Russian foreign ministry said.
Meanwhile US Republican presidential hopeful Newt Gingrich said Sunday the United States could take covert action to help oust Assad, without using US troops.
Gingrich, who is struggling to keep up with frontrunner Mitt Romney in the Republican race, told the CBS programme "Face the Nation" that Washington should act to help remove the Syrian leader blamed for a deadly crackdown on opponents.
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"I think there are a lot of things we could do covertly in terms of supplying weapons, supplying -- helping people in the region supply advisers," the former House speaker said.
The Russian and Chinese vetoes came hours after the opposition Syrian National Council (SNC) reported a "massacre" overnight Friday in the central flashpoint city of Homs with more than 230 civilians killed during an assault by regime forces.
On Sunday, activists reported more shelling in the city, with at least 56 civilians and 28 regular army troops killed the day after 48 people were reported dead.
Army deserters destroyed a military control post in the northeast village of Al Bara, killing three officers and capturing 19 soldiers, the Syrian Observatory for Human Rights said Monday.
The weekend death toll was one of the bloodiest since the uprising against Assad's regime erupted almost 11 months ago.
Opposition groups say at least 6,000 people have now been killed.
The second UN double veto in four months also fuelled fears among Syrian activists of a new surge of violence that would once again target Homs.
"The SNC holds Russia and China accountable for the escalation of killings and genocide, and considers this irresponsible step a licence for the Syrian regime to kill," it said in a statement.
In Libya, crowds of Syrians chanting anti-Russian slogans entered Moscow's Tripoli embassy and replaced the Russian flag with the new Syrian flag while hundreds protested outside the Russian embassy in Beirut.
And Turkish police fired tear gas to disperse protesters seeking to storm the Syrian consulate in Istanbul.
Iran, however, welcomed the veto on the resolution condemning its ally Syria and accused the Security Council of attempting to interfere in the country's internal affairs
Assad's troops shelled Homs overnight Friday, killing at least 260 civilians, the SNC said, while the Syrian Observatory for Human Rights said about 100 women and children were among its toll of 237 dead.
The tolls could not be independently confirmed. Damascus denied responsibility, blaming the deaths on rebels seeking to swing the UN vote.
The UN resolution -- approved by 13 of the 15-member Security Council -- was proposed by European and Arab nations to give strong backing to an Arab League plan to end the crackdown.
On Sunday, League chief Nabil al-Arabi said the bloc would press on with mediation efforts to find a political solution and avoid foreign intervention in Syria.
Syrian government mouthpiece Tishrin called the veto "a catalyst" and said it would help accelerate reforms in the country.
Tunisia urged other Arab nations to follow its lead after it said on Saturday it was expelling Syria's ambassador and withdrawing its recognition of the Assad government.