Kofi Annan and Ban Ki-moon head for Russia and China on Monday to press the two UN Security Council doubters to back tougher action against President Bashar al-Assad to halt the slaughter in Syria.

The visits by the UN-Arab League envoy and the UN leader come at a crucial new stage in the 16-month old conflict. The Security Council has until Friday to renew the UN mission in Syria but is divided over Western calls to add sanctions.

"So divided that maybe Annan and Ban now have the most influence over Russia and China to get anything done," said one senior UN council diplomat, speaking on condition of anonymity.

Russia has led the resistance and Annan is to meet President Vladimir Putin and Foreign Minister Sergei Lavrov during two days of talks in Moscow, said his spokesman Ahmad Fawzi.

Russia's reluctance to act against its Damascus ally has seen it hold up a UN Security Council statement condemning the massacre of more than 150 people in the village of Treimsa on Thursday.

A draft statement which said the Syria government is in "violation" of its international commitments was circulated among the 15 council nations on Friday, diplomats said. Russia's envoys said they could not agree without approval from Moscow.

China has supported Russia's rejections and Ban heads for Beijing on Monday, officially for a China-Africa summit. But Syria will top his talks agenda when he meets President Hu Jintao, Premier Wen Jiabao, Foreign Minister Yang Jiechi and other leaders, said a UN official.

Ban has said that international inaction on Syria would be giving "a license for further massacres." He already urged China to use its "influence" to back Annan's peace plan when he spoke with Yang by telephone on Saturday, said UN spokesman Martin Nesirky.

Diplomats said China has mainly opposed action because of its strong alliance with Russia on the Security Council, where the two normally vote together.

"The problem here is Russia," said a UN Council diplomat, speaking on condition of anonymity.

"Their credibility on this issue is shot to pieces. Now I am not saying they are not working behind the scenes, but clearly it has not worked. And they have to admit that they have not been pushing Assad hard enough or they have and they failed to persuade him," the diplomat said.

Annan has called on the Security Council to order "consequences" for any failure to carry out his six-point plan which Assad has agreed but failed to carry out. The plan includes the withdrawal of heavy weapons.

Britain, the United States, France, Germany and Portugal want a resolution passed this week that would threaten sanctions if Assad does not pull back his main weapons.

Russia, which has proposed a resolution which just renews the UN mission, has said the threat is "unacceptable" and would not be allowed. Russia and China have twice used their powers as permanent members of the Security Council to veto resolutions which just spoke of possible measures.

Moscow has also said that the Security Council cannot force Assad to stand down as part of any solution and that more pressure must be put on the opposition.

Diplomats have warned that the council battle over Syria could see the UN mission closed down if there is no agreed resolution.

The Western nations have said there is little point in keeping the nearly 300 unarmed observers in Syria if there is no ceasefire or a political process to monitor.

Meanwhile, UN observers who returned from the village of Treimsa on Sunday said that, based on what they saw and on witness accounts, the attack there was targeted at army defectors and activists.

Staff from the UN Supervision Mission in Syria (UNSMIS) observed over 50 houses that were burned or destroyed, and "pools of blood and brain matter were observed in a number of homes," a UN statement said.

"According to those interviewed, the army was conducting house to house searches asking for men and their ID cards," the document noted. "They alleged that after checking their identification, numerous were killed. Other men were taken out of the village."

But the UN mission said that the number of casualties suffered at Treimsa was "still unclear."