Leading article: Bloodshed that demands a concerted UN effort

What will it take to persuade Russia that the appalling situation in Syria has gone too far? It is no longer just a matter of the 5,500 or more people who have been killed by the brutal Assad regime, although by any normal measure that would be reason enough.

Now, after almost a year, the conflict is rapidly turning into a full-scale humanitarian disaster. With swathes of the city of Homs under almost constant bombardment, civilians are running out of food, water and medical supplies, those displaced by the shelling crowding together in mosques for shelter.

That the International Committee of the Red Cross has broken its habitual silence to call publicly for a ceasefire is testament to both the desperate straits to which innocent Syrians are being reduced and the obduracy of President Bashar al-Assad.

President Assad's obstructions raise the possibility that he is breaking Geneva Convention rules on ICRC access to conflict zones. As part of its effort to bring all possible pressure to bear on the blood-stained regime, the international community must pursue the legal implications forthwith. Similarly, it is time for the United Nations to refer President Assad to the International Criminal Court for the crimes against humanity that certainly appear to be being perpetrated by his government's forces.

The only chink of light in the almost uniformly dark outlook for Syria is that Russia has now lent its support to the ICRC's plea for a daily two-hour ceasefire. But, although any break in the violence would clearly be better than none at all, two hours each day can hardly be considered enough. Neither does Moscow's backing of the aid agency's efforts free it from the moral obligation to force its indefensible ally in Damascus to the negotiating table.

At the Friends of Syria conference in Tunis tomorrow, representatives from more than 70 countries will discuss ways to ensure help reaches the worst-affected areas, options for further sanctions, and how best to provide assistance to the splintered Syrian opposition.

Such efforts are, of course, welcome. But talk of arming the regime's opponents should be restrained. While it may be politically expedient to keep the option on the table, putting it into practice carries the risk of helping Syria along the path to sectarian conflict, even more so given the alarming signs that it is becoming a magnet for extremists from elsewhere in the region.

The notable absence from tomorrow's summit, however, is Russia. And by holding out in support of its long-standing alliance with President Assad – and his even bloodier father – Moscow remains the single biggest obstacle to ending the bloodshed. Such a stance cannot continue. Standing by a vicious regime is one thing; playing handmaiden to a humanitarian crisis must, surely, be another.

Russia – and, to a lesser extent, China – used Moscow's nascent attempts to broker a peace deal as part of its rationale for blocking the Arab League-backed UN resolution on Syria earlier this month. Given the complete absence of any constructive results from the much-vaunted talks, the UN must press its advantage and return to the issue of a formal resolution with all possible haste.

Basic humanity calls for the condemnation of the Syrian regime and the immediate end to the violence. With the situation deteriorating daily, and fears of a ground assault on Homs that could end in a massacre, there is no time to waste. Marie Colvin, the leading British journalist killed in the shelling of Homs yesterday, wondered, on the day before her death, how the world could stand by. It is a question that the UN must force Russia to answer.