Leading article: Russia must act now to halt Assad's slaughter

That something utterly appalling happened outside the Syrian city of Houla on Friday is beyond doubt.

As the sickening pictures of murdered children showed – pictures rightly reprinted by several British newspapers, including our sister paper The Independent on Sunday – many victims were children, at least some of whom had had their throats cut. Even as the Syrian authorities denied responsibility, blaming Islamists and terrorists, they conceded that at least 90 people had been killed. Of these more than 30 were children, slaughtered, as the pictures attest, in cold blood. Opposition activists accused pro-regime gunmen of the massacre.

The killings near Houla constitute the worst single incident in 14 months of often violent unrest in Syria. And while the full truth of it has yet to be told, the murderous spree seems to have followed the regime's use of heavy weapons against protesters in the town, which is close to the heart of anti-regime resistance in Homs. Any such action by the forces of President Bashar al-Assad would have been in flagrant violation of the six-week-old ceasefire brokered by the UN and the Arab League.

The massacre that came next, which may be classed as a war crime, could well mark a turning point in the conflict, reinvigorating opposition forces that had appeared to be fragmenting and reminding the world of the new horrors to which the Assad government could resort in its struggle to retain power. The activists who posted the pictures online were clear that they saw them not just as evidence of crimes, but as a rallying cry that could shame international opinion into providing them with not just moral, but military support.

At which point the argument becomes more complicated. For all the expressions of outrage from Western leaders and the calls for something to be done – an urgent meeting of the UN Security Council (Britain), a summons to the Friends of Syria group (France), "intensified pressure on Assad and his cronies" (the US) – direct military intervention, as hoped for by Syria's opposition, is unrealistic.

Syria is not Kosovo, nor is it Libya. It is a big country in a highly volatile neighbourhood and is slipping ever closer to all-out civil war. While there are entrenched areas of resistance, the opposition itself has been plagued by splits and by no means all Syrians are convinced of the need to remove the Assad clan. Any change of power needs to be supported and sustained by Syrians themselves.

This is not to say that foreign governments can, or should, do nothing. One track runs through Moscow. By fortunate chance, the Foreign Secretary, William Hague, is visiting Russia today and Syria is to be added to the long list of bilateral issues on his agenda. Russia holds the key to tougher international action on Syria. After notoriously rejecting an earlier resolution, it voted for Kofi Annan's mission, and it has at times seemed ready to countenance a Syria without Assad. Still, Syria remains one of its biggest customers for arms; Moscow has not signed up to the sanctions imposed by the US and the EU, and Russian officials have avoided condemning the regime in public.

If any one country can wield influence in Syria, it is Russia. But its co-operation is also crucial if the international community is to show a united front, which is essential if anything is to change. In present circumstances, the temptation will be to write off the UN process and Kofi Annan's six-point plan. But it has not been exhausted. While there have been violations of the ceasefire aplenty, of which the Houla massacre is by far the most egregious, the ceasefire did bring some diminution of the violence and might have brought more, had there been more observers than the 260 now there. Massively beefing up this operation should be the priority.