The UN Security Council is expected to vote later on Saturday on a resolution backing an Arab League peace plan for Syria, amid reports of more than 200 people killed by shelling in Homs.
The draft resolution calls on President Bashar al-Assad to hand power to a deputy to oversee a transition.
Russia has objected to earlier versions of the text and it is not clear how it will vote, or if it will abstain.
Meanwhile, Syrian activists described violence in Homs as "a real massacre".
The UK-based Syrian Observatory for Human Rights said Syrian forces were firing mortars into the district of Khalidiyya and some buildings were set on fire. It said women and children were among the dead.
Rami Abdulrahman, head of the observatory, called for the "immediate intervention" of the Arab League to end the violence.
The reports cannot be independently verified. However, footage broadcast on the Al-Jazeera and Al-Arabiya TV channels showed dozens of bodies on the ground.
The BBC's UN correspondent Barbara Plett says momentum for the vote has been building in recent days.
Russia - Syria's strongest ally on the council - has objected to parts of the latest draft but did not put forward any revisions, so a vote was called for 09:00 EST (14:00 GMT) on Saturday.
Moscow has previously vetoed UN resolutions on Syria but there is a feeling now that things could be different, our correspondent adds.
On Thursday, Russian UN Ambassador Vitaly Churkin told diplomats he would veto the resolution if it were put to a vote on Friday in its current form.
It is believed Moscow objects to the phrase which says the council "fully supports" the Arab League plan.
On Friday, the Russian Interfax news agency quoted Deputy Foreign Minister Gennady Gatilov as saying that the draft was "not enough for us to be able to support it in this form".
However, by Friday evening the Russian delegation had not presented any new wording, diplomats said.
Moscow has continued to supply weapons to Syria despite the uprising there.
In another development, a group of about 20 protesters - thought to be of Syrian origin - forced their way into the Syrian embassy in Berlin late on Friday and damaged offices, police said.
German TV showed a Syrian resistance flag hanging from one of the windows and graffiti sprayed on the outside of the building.
The intruders were briefly detained as they left the embassy. The German foreign ministry condemned the protest and apologised to the Syrian ambassador, who had been in the building at the time.
Syria has been gripped by nationwide protests against President Assad's regime for almost a year.
The UN stopped estimating the death toll after it passed 5,400 in January, saying it was too difficult to confirm numbers.
The Syrian government says at least 2,000 members of its security forces have been killed fighting "armed gangs and terrorists".