SOFIA, Bulgaria — US Secretary of State Hillary Clinton vowed Sunday to bolster existing sanctions against the Syrian regime and seek further ones to block funding and arms shipments to Damascus.

Seeking other ways to turn the screws on Assad a day after Russia and China vetoed a resolution on Syria, Clinton said Washington will also work with Syria's friends worldwide to support the peaceful aims of the opposition.

"What happened yesterday at the United Nations was a travesty," Clinton said during a visit to Bulgaria following her failed talks in Munich with Russian Foreign Minister Sergei Lavrov.

"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 press conference with Bulgarian Prime Minister Boyko Borisov.

On the sidelines of the Munich Security Conference, Clinton said Saturday she had tried to bridge differences over the UN resolution during a 45-minute meeting with Lavrov.

"I thought there might be some ways to bridge even at the last moment a few of the concerns that the Russians had," she said. "That has not been possible."

In Sofia, she said the 13 of the 15 Security Council members who backed the resolution sought to start a "process for political engagement that would lead to a transition" to a new democratic Syria.

"We feared that failure to do so would actually increase the chances for a brutal civil war," she said, recalling that Syrians were beginning to arm themselves against the crackdown.

"Faced with a neutered Security Council we have to redouble our efforts outside of the UN with those allies and partners who support the Syrian people's right to have a better future," Clinton said.

"We have to increase diplomatic pressure on the Assad regime and work to convince those people around President Assad that he must go and that there has to be a recognition of that and a new start," she said.

"We will work to seek regional and national sanctions against Syria and strenghten the ones we have," she said.

"They will be implemented to the fullest to dry up the sources of funding and the arms shipments that are keeping the regime's war machine going," she said.

Analysts believe that Russia fears Assad's departure would cost Moscow hundreds of millions of dollars in arms contracts, as well as its last remaining ally in the region after Libyan leader Moamer Kadhafi was ousted in the Arab Spring.

"We will work to expose those who are still funding the regime and sending it weapons that are used against defenceless Syrians including women and children," Clinton said.

The veto came hours after the opposition Syrian National Council (SNC) reported a "massacre" overnight Friday in the central flashpoint city of Homs.

Activists and residents said more than 230 civilian, including women and children, were killed during an assault by regime forces.

"We will work with the friends of a democratic Syria around the world to support the opposition's peaceful political plans for change," Clinton said.

On Sunday, Arab League chief Nabil al-Arabi said the bloc would press on with mediation efforts to find a political solution in Syria to avoid foreign intervention in the country.