A flood of military reinforcements has been a prelude to previous offensives by President Bashar Assad's regime, which has tried to use its overwhelming firepower to crush an opposition that has been bolstered by defecting soldiers and hardened by 11 months of street battles.
"The human loss is going to be huge if they retake Baba Amr," said Rami Abdul-Rahman, who heads the Britain-based activist group Syrian Observatory for Human Rights.
Russia's U.N. Ambassador Vitaly Churkin said Russia will put forward a proposal at the U.N. Security Council in the coming days regarding humanitarian aid to Syria, the Itar-Tass news agency reported.
Churkin was quoted by the agency as telling Vesti 24 television on Monday that the council "could undertake concrete steps aimed at solving humanitarian issues, relying, among other things, on the fact that Damascus allowed the Red Cross to bring humanitarian aid to some regions."
Churkin gave no details about the proposal, his spokesman Anton Uspensky told AP. Russia and China have vetoed two Security Council resolutions backing Arab League plans aimed at ending the conflict and condemning the government crackdown.
The central city of Homs -- and in particular the opposition district known as Baba Amr -- has become a critical ground for both sides.
The opposition has lionized it as "Syria's Misrata" after the Libyan city where rebels fought off a brutal government siege. Assad's regime wants desperately to erase the embarrassing defiance in Syria's third-largest city after weeks of shelling, including a barrage of mortars that killed up to 200 people earlier this month. At least nine people were killed in shelling Monday, activists said.
Another massive death toll would only bring further international isolation on Assad from Western and Arab leaders.