Syrian forces killed at least 200 people and wounded hundreds more in a barrage of shelling early on Saturday in Homs, according to several news reports.
Meanwhile, in a move that US Ambassador to the UN Susan Rice said Washington said had "disgusted" Washington, Russia and China vetoed a UN Security Council resolution condemning the violence in Syria and calling for president Bashar al-Assad to step down.
(GlobalPost reports: Russia, China veto UN Security Council resolution on Syria)
The death toll from the offensive, in one of the main flashpoints of opposition to President Bashar al-Assad's regime, if confirmed would amount to the single deadliest attack of the nearly 11-month-old uprising, the Washington Post wrote.
The assault reportedly began with military forces firing shells and rockets on the neighborhood of Khaldiyeh late Friday evening, then spread to the Baba Amr and Bab al-Sebaa neighborhoods, the paper reported, citing activist Omar Shakir.
Shakir, speaking by telephone from Homs, said he heard hundreds of missiles strike the area. Rockets crashing into neighborhoods and whole buildings collapsed on top of residents.
Information to media outlets came from the British-based Syrian Observatory for Human Rights and the Local Coordination Committees, citing witnesses on the ground.
"The death toll is now at least 217 people killed in Homs, 138 of them killed in the Khaldiyeh district," Rami Abdulrahman, head of the British-based group, told Reuters, citing witnesses.
‘‘This is the worst attack of the uprising, since the uprising began in March until now,’’ he reportedly said.
(GlobalPost reports: Syria: Mass killings uncovered near Damascus)
Syria's government denied the assault, however. The Associated Press cited Syrian TV as saying the reports were untrue and came in the context of "incitement by the armed groups" against Syria to be exploited at the Security Council.
The reported attack came ahead of a UN Security Council vote, vetoed by China and Russia, which condemned the government’s violent crackdown on anti-regime protests.
US Ambassador to the United Nations Susan Rice said Washington was "disgusted" by the vetos.
More from GlobalPost: Russia, China veto UN Security Council resolution on Syria
The UN estimated in January that at least 5,400 people had been killed, including soldiers who defected and those who refused orders to fire on civilians, CBS reported. The UN has refrained from updating its tally because of the difficulty of verifying figures.
The US and its partners have ruled out military action but had wanted the UN to endorse an Arab League plan that calls on Assad to hand power over to Syria’s vice president, according to the AP.
Meanwhile, anti-Assad rallies broke out outside Syrian embassies in Britain, Germany and the United States after the Homs killings were reported.
According to Reuters, protesters also stormed the Syrian embassy in Cairo, smashing furniture and equipment and setting fire to parts of the building in protest over the latest bloodshed.