Syria is slipping closer to all-out war with each passing day, the UN peace envoy Kofi Annan has warned.
Mr Annan, in Doha for a meeting with Arab League members, warned that the conflict was developing an alarming sectarian dimension.
Qatar urged the UN to set a deadline for Mr Annan's peace plan, saying there should be no more "stalling".
Syrian opposition groups said Russia was becoming part of the problem by supporting President Bashar al-Assad.
"The spectre of all-out civil war, with a worrying sectarian dimension, grows by the day," Mr Annan said.
He said that the massacre at Houla, in which 108 people were killed, many of them women and children, was appalling and that the crisis was now at "tipping point".
The UN Human Rights Council approved an investigation into the mass killing last week.
Mr Annan, who admitted that his six-point plan had failed to stop the violence, expressed frustration with the Syrian president for not matching words with actions.
He urged the international community not to forget that the conflict risked spilling over.
It was vital, he said, "to keep our goals firmly in view: stopping the killing, helping the suffering population, securing a political transition - and, I would add, ensuring that the crisis does not spread to the neighbours".
He said it was crucial that the international community moved together.
Burhan Ghalioun, of the Syrian National Council - the divided body which says it represents the political opposition to Mr Assad - said Russia was standing in the way of progress.
"With its support of the regime and for Assad remaining, Russia has become part of the problem rather than part of the solution," he said.
"If it co-operates to find a formula that makes Assad leave, it will become part of the solution."
Russia's President Vladimir Putin has resisted diplomatic pressure to support tougher action against Syria's government, calling for more time to be given to Mr Annan's plan.
The country voted against the recent US-backed human rights council resolution, arguing that it was "unbalanced".
Meanwhile, Prime Minister Sheikh Hamad bin Jassim al-Thani of Qatar said a time limit should be set for Mr Annan's mission - which began three months ago.
He also called for it to fall under Chapter 7 of the UN charter, enabling the use of force.
"We, the international community, cannot accept the situation to continue as it is," he said.