Efforts to forge a united international response to the Syria crisis have been dealt a double blow as the West rebuffed a proposal to work with Iran and Russia rejected a new US peace initiative.

With violence escalating in the wake of this week´s massacre of 78 Sunni villagers near the city of Hama, Kofi Annan, the international envoy to Syria met Hillary Clinton, the US Secretary of State, to try to salvage his disintegrating ceasefire plan.

At least 20 people were killed on Friday as rebels stepped up a week- long offensive to avenge the killings in Qubair, the fourth incident of mass bloodletting directed at members of Syria´s Sunni majority in a fortnight.

Their operations included one of the most ferocious assaults on Damascus, the capital city, in months.

With demonstrations erupting across the country, the regime responded to the mounting volatility by shelling the city of Homs, one of the most-bitterly contested battlegrounds of the 15-month uprising against President Bashar al-Assad.

The latest killings have galvanised Western powers into launching a fresh attempt to save Syria sliding into sectarian war between the country´s Sunni majority and the president´s Alawite Shia sect.

Britain, France and the United States said they were working to introduce a United Nations Security Council resolution imposing sanctions on the Syrian government that would be binding for all member states.

Previous such attempts have been blocked by Russia and China. At the same time, Mr Annan has responded to Western warning that they were close to writing off his plan, which came into force in April, by acknowledging the need for major modifications to save it from total failure.

He has suggested forming a "contact group" bringing together Western and Middle Eastern states, as well as Russia and China, in the hope of ending the international rift over Syria and paving the way for Mr Assad´s departure.

But a row immediately erupted after he proposed to include Iran in the group, justifying the move on the grounds that it was "an important country in the region" whose involvement was key to bringing about a solution to the conflict.

Western powers categorically rejected the notion, with Mrs Clinton accusing Iran of "helping to stage-manage the repression" in Syria in order to prop up Mr Assad, a fellow Shia and a key ally of Iran. Britain and France were similarly vocal in their opposition.

"Iran is a country which is supporting some of the unacceptable violence and supporting the Syrian regime in what it´s doing to the Syrian people," Alistair Burt, the Foreign Office minister responsible for the Middle East, said.

Mrs Clinton suffered a major setback of her own as Russia rebuffed her pleas to support a regime change in Damascus.

Hoping to exploit Moscow´s growing unease over its proximity to Mr Assad, she tried to win Russian support for a US initiative to end the violence in Syria built around the Syrian leader´s departure. To make the proposal more palatable, she told Russian officials that Mr Assad´s resignation would not be a condition for a settlement in Syria but rather its ultimate goal.

Although Russia has distanced itself from Mr Assad, its fundamental position on the crisis has not changed and Mrs Clinton´s top Syria aide, Fred Hof, left Moscow empty-handed.

Russia and China, which say the Assad regime and the rebels are equally responsible for the violence, confirmed that they would thwart any attempt at forcing Mr Assad from office. In a blow to Western hopes, both states also signalled their continuing opposition to UN sanctions.

Meanwhile in Syria itself, explosions and gunfire could be heard from the heart of Damascus as clashes erupted in three of the city´s suburbs. The battles were among the fiercest of the uprising.

Fighting was reported across the country, but a government artillery offensive on the rebel-held district of Khaldiyeh in Homs will alarm the international community the most.

Opposition activists in the city said that shells were falling on the area at more than five a minute, raising fears that government forces were preparing to launch an infantry assault to retake the district. (© Copyright of Telegraph Media Group Limited 2012. 06/09/12)