ARAB AND WESTERN governments plan to mount a major diplomatic offensive at the United Nations on Tuesday in the hope of breaking a deadlock in the Security Council on Syria. Nabil ­el-Araby, secretary general of the Arab League, will present a plan to end the swelling violence by calling for dictator Bashar al-Assad to step down and be replaced by a coalition government that would organize elections. Secretary of State Hillary Rodham Clinton and the foreign ministers of Britain and France will attend the meeting as a way of raising the pressure on Russia, which has been blocking action by the Security Council since the Syrian uprising began 10 months ago.

For now, there is little sign that Russia will yield: The government of Vladi­mir Putin is insisting that it will not support the removal of a regime that has been its primary ally in the Middle East. Yet if sustained, Moscow’s intransigence is likely to precipitate a disaster, both for Syria and for itself.

The Arab League plan is probably the only means left to avoid a full-scale civil war in Syria. Since the failure of the league’s observer mission last week, violence has again accelerated, while moving to the edge of Damascus. As a statement issued by Ms. Clinton reported Monday, “intensified Syrian security operations all around the country . . . have killed hundreds of civilians” in the past few days. It added: “The government has shelled civilian areas with mortars and tank fire and brought down whole buildings on top of their occupants.”

Despite such brutal behavior, Mr. Assad’s forces seem to be losing ground to armed opposition groups, which briefly held control of several Damascus suburbs and appear to be entrenched in cities such as Homs and Hama. The assessment of most outside observers is that the Assad regime is doomed. That means that if Russia continues to prop it up, it will not only damage its position with other Arab governments but will endanger its assets in Syria — including a naval base and weapons sales.

Though Arab and Western countries will welcome Mr. Assad’s ouster, the means by which it happens are crucial. A managed transition, like that outlined in the Arab League plan, could end the bloodshed relatively quickly and give the upper hand to secular and pro-democracy forces. The longer the fighting goes on, the greater the chance that Syria will be overtaken by a merciless sectarian war between the majority Sunni and minority Alawite communities, with Christians and Kurds caught in between. That would empower Islamic extremists, and it could trigger renewed sectarian conflict in Iraq and Lebanon.

As long as it has Russia’s diplomatic and material support, the Assad regime is more likely to hold together. That’s why the high-level lobbying campaign at the Security Council is important, and it’s why the Obama administration should place Russian cooperation on Syria at the top of the bilateral agenda with Moscow. At the same time, Western and Arab governments must consider other means of speeding an end to the Syrian conflict. Military intervention may be off the table for now, but non-lethal material aid for the opposition should not be.