It is no use warning of impending civil war in Syria; that has already begun. The bombardment of Homs is just the latest escalation in a conflict that has claimed more than 5,400 lives since the first stirrings of revolt against President Bashar al-Assad last March. As the Foreign Secretary said as he announced the recall of the British ambassador from Damascus yesterday, the situation is utterly unacceptable and demands an international response.

But if the accelerating death toll is sickening, perhaps more appalling still is that the surge in government-sponsored violence is so predictable. Russia and China, by vetoing an Arab League-inspired UN resolution on Syria, have freed President Assad from any restraint. Indeed, the violence in Homs in recent days – with fears of a full-scale military assault to come – is a direct result of their unforgivable self-interest. Ostensibly, they rejected the draft resolution on the grounds that it focused too much on the conduct of the Assad regime, without an equal requirement for the opposition forces to eschew violence. Such a claim is shamefully disingenuous.

Ahead of the UN meeting last week, there was little expectation of support from China, given both Beijing's interest in defending governmental autonomy and the generalised desire to check the powers of what it sees as the overweening West. An abstention from Russia, however, might have tipped the balance. Instead, Moscow has abandoned the Syrian people to the depredations of a regime that is daily becoming more murderous.

In part, Russia's obstinacy stems from a sense of betrayal over Libya, where a UN-backed no-fly zone turned into active military support for the rebels. In part, it comes from an understanding of the Arab Spring as a dangerous shift in the balance of the regional sectarian power of Syria and Iran, on one side, and Sunni Arab monarchies on the other. But more than anything, it rests on a long-held alliance, stretching back to President Assad's father, and now including the use of the Tartus naval base and $5bn-worth of weapons contracts.

Russia still has the chance to put its immense influence in Syria to good use. Sergei Lavrov, the Russian Foreign Minister, will be in Damascus today as part of a Kremlin demarche, supposedly to broker peace talks. Thus far, Russia's efforts have been hardly more than diplomatic window dressing. It can only be hoped that the hit to Moscow's international standing that has followed the UN veto will focus minds. Sadly, it is more likely to confirm Russia's sense of aggrieved isolation.

The debacle at the Security Council certainly throws the limitations of UN diplomacy into sharp relief. It is even arguable that, by revealing the dissension so publicly, the failure has given President Assad a free hand.

There are still options available that involve neither turning backs on the humanitarian horror nor leaping to the dangerous – and erroneous – conclusion that the only effective response is a military one. Most immediately, efforts continue to maintain the momentum behind the Arab League plan for a government of national unity and fresh elections. Rightly so. Steps to toughen sanctions against the regime to stop it acquiring more weapons for slaughter must also move ahead rapidly.

The closure by the US of its embassy in Damascus signals Washington's view that there is little now to be gained from talking to President Assad. Yet, further pressure – be that through formal recognition of the opposition, or the suspension of Syria from UN bodies – can and must be brought to bear on Damascus.

Hillary Clinton described the vetoing of the UN resolution as a "travesty". She is right. But this cannot be the international community's last word.