Diplomatic crisis follows day of continued violence in which at least 50 people were killed in Homs, according to activists

Britain and the US recalled their ambassadors to Damascus on Monday in protest at what the British foreign secretary, William Hague, called the "doomed" and "murderous" regime's violent behaviour towards its civilian population.

The diplomatic crisis followed a morning in which at least 50 people were killed in attacks on the Syrian city of Homs, according to activists, including the bombardment of a field hospital in which 19 people were killed.

Speaking in the House of Commons, Hague said of Assad's government: "There is no way it can recover its credibility internationally or with its own people."

Hague recalled Britain's ambassador in Damascus, Simon Collis, to London for "consultations" on what he termed an "utterly unacceptable situation which demands a united international response".

The foreign secretary also signalled the west would now scramble to explore alternative, non-UN routes in an attempt to halt the killing in Syria and prepare for a post-Bashar al-Assad future. As well as continuing support for the Arab League, Hague said the UK would intensify its contact with the Syrian opposition, and would back a new Arab-led group, Friends of Syria. "Britain will be a highly active member in setting up such a group with the broadest international support," he said.

He described Russia's and China's vetoes of the United Nations security council resolution censuring Syria as a grave error of judgment and a betrayal that implicitly "left the door open" to further human rights abuses.

Earlier in the day, an unrepentant Russia accused the international community of "hysteria" following global condemnation of Moscow and Beijing's decision to veto the UN resolution on Syria.

Russia's foreign minister, Sergei Lavrov, dismissed the reaction from "some western voices" as "verging on the hysterical" and called it an "indecent" attempt to pin blame for the out-of-control violence in Syria "on one side only".

The US, Britain, France and Germany all expressed disgust at Moscow's action.

The US closed its embassy in Damascus and evacuated its ambassador and other diplomats amid security concerns. The French president, Nicholas Sarkozy, described the rejection of the Arab-backed security council resolution as a scandal, with the German chancellor, Angela Merkel, saying she was appalled.

Hague's remarks came amid another day of bloodshed and mayhem in Homs, the opposition-controlled town relentlessly targeted by Damascus since Friday. At least 50 people were killed on Mondaywhen shells slammed into a makeshift clinic and residential suburb, in the third day of indiscriminate bombardment by the Syrian army, activists said. Another 10 people were reported killed elsewhere, they added.

The Guardian was unable to independently verify the casualty figures.

The government denies shelling Homs. But activists say as many as 200 people were killed on Saturday, the highest death toll since the uprising in Syria began last March. Arab satellite television stations broadcast live footage from the town, showing smoke rising from some buildings and explosions.

In an interview with NBC, Barack Obama said that despite the failure of UN diplomacy there was no prospect of western military intervention in Syria. But he said he still believed it was possible to reach a negotiated solution to the conflict. He added: "The Assad regime is feeling the noose tightening around them. This is not going to be a matter of if, it's going to be a matter of when."

Asked why Syria differed from Libya, Obama said there was a lack of unity among the major powers in dealing with Syria. He stressed, however: "We have been relentless in sending a message that it is time for Assad to go, that the kind of violence we've seen exercised against his own people over this weekend and over the past several months is inexcusable."

Lavrov will on Tuesday lead a Russian diplomatic mission to Damascus and hold talks with Assad, Syria's president. Mikhail Fradkov, Russia's foreign intelligence chief, will also attend. There has been speculation Moscow may privately be seeking to persuade Assad to make a "controlled exit", handing over power to trusted senior generals, in a move that would preserve Russia's influence in a post-Assad scenario.

Russia has cast its efforts as an even-handed attempt to get both sides to negotiate, in contrast to the partial diplomacy of the west. Lavrov said he would urge Assad to withdraw his heavy weapons from Syria's towns and cities, a key Arab League demand. But he also said he wanted what he called "extremist groups" – opposition fighters from the Free Syrian Army – to disarm as well.

Analysts said Russia's diplomatic initiative stood little chance of success, with Assad emboldened by the Russian and Chinese votes to crush the rebels militarily, and Russia's credibility with Syrian opposition groups at zero. "My gut feeling is it will go nowhere," David Hartwell, senior Middle East analyst at IHS Jane's said.

He added: "The debate has been sharpened by what happened on Saturday. Moscow's argument that Assad is a credible figure who can lead a reform movement in Syria is increasingly weak. The Russians have got themselves in a situation where they are not treated seriously by anybody, certainly not by the opposition."

Russia appears to have rejected the UN resolution for several reasons. The Kremlin has traditionally enjoyed good relations with Syria and supplies it with billions of dollars worth of military hardware. In return, Damascus gives Russia a strategic foothold in the Middle East, allowing it the use the Syrian port of Tartus as a naval base. Russia is also Syria's third biggest trading partner (after Ukraine and China).

But geopolitics also played a role. The Kremlin is keen to create difficulties for the west, and the US in particular. It is happier siding with a fellow authoritarian regime, especially one in the grip of a popular uprising. Additionally, Moscow feels betrayed after supporting last year a UN no-fly zone in Libya, which, it says, was used as a pretext for western-engineered regime change.

And then there are domestic factors, ahead of next month's presidential "election" in Russia and unprecedented street protests against Vladimir Putin's rule. Putin's is reflexively opposed to what he sees as US hegemony and western meddling in sovreign states. "The Russians think Assad's days are over and they are thinking about how to safeguard their position in the region," Ghassan Ibrahim, a Syrian dissident based in London, told Reuters. "Syria is their only door into the region and it gives them influence. They need to protect it. But do they have enough power to manipulate Assad (to step down)?"

Writing in the Rossiskaya Gazeta, Russia's former prime minister Yevgeny Primakov on Monday suggested the UN security council resolution was part of a western conspiracy. Its ultimate aim, he suggested, was to remove the Assad regime in Syria so as to isolate Iran, which the US believes is seeking to develop nuclear weapons.

"The United States and its NATO allies want to exploit the situation that arose in the spring of 2011 in the Arab world with the aim of getting rid of Arab regimes it dislikes," Primakov - a veteran of previous Russian "peace initiatives", including to Saddam Hussein in 1991 - said.

The inclusion of Fradkov, the head of Russia's SVR foreign intelligence agency, in Russia's mission is intriguing. Leaked WikiLeaks cables describe him as a leading member of Russia's security elite, and a "pragmatic hardliner who shares a world view of Soviet xenophobia and distrust of the west."

The US, meanwhile, said it had closed its embassy in Damascus over what it said were security concerns.The US state department said that the Syrian government was informed that the embassy had been closed and the American ambassador, Robert Ford, and the 17 staff remaining in Damascus had left the country only after all of them had crossed into neighbouring Jordan by road.

US officials say the Syrian security forces are so stretched by the uprising that they are not able to sufficiently protect the embassy. Those concerns were heightened by two car bomb attacks on state security offices in Damascus last month.