The UN Security Council is to meet shortly to discuss the killing of at least 90 people, including 34 children, in Syria, diplomats say.
The massacre has been widely condemned by world leaders.
The UK wants Russia, Syria's only major foreign ally, to put pressure on President Bashar al-Assad to halt civilian deaths.
The Syrian government has denied any involvement in the killings in the town of Houla, blaming "terrorists".
The meeting was called after Russia rejected a joint UK-French statement condemning the killings, diplomats say, and is due to start at 1830GMT.
Russia is said to want a briefing from the head of the observer mission in Syria, Maj Gen Robert Mood.
According to Reuters, a Russian deputy foreign minister said the "tragic" events in Syria deserved condemnation and that he would press for a UN assessment about what happened.
Earlier, Gen Mood told the BBC that "the circumstances leading to the killing are still unclear" but are being investigated by UN monitors in Houla.
He also said that more than 34 children had now been confirmed as having died in Houla - up from the previous figure of 32.
Syria's foreign ministry spokesman Jihad Makdissi said there was a clash in the area on Friday afternoon between security forces and armed terrorists.
He said hundreds of armed men then gathered, armed with heavy weapons including mortars and anti-tank weapons, and attacked Houla.
Activists, however, say the Syrian military bombarded Houla after demonstrations. They say that some of the victims were killed during the shelling, while others were summarily executed at close range by the regime militia known as the "shabiha".
The White House said it was horrified by the credible reports of the massacre.
"These acts serve as a vile testament to an illegitimate regime that responds to peaceful political protest with unspeakable and inhuman brutality," a White House spokesman said.