Syrian government forces launched a mortar and rocket assault on the country’s third-largest city Friday night that activists said killed more than 200 people, intensifying pressure on the U.N. Security Council as it prepares to vote on a measure aimed at ending a bloody government crackdown.

If the death toll is confirmed, the military assault on Homs would be the single deadliest attack of the 10-month-old uprising that has devastated Syria.

The Security Council vote, which comes after months of swelling international outrage over the crackdown, is expected Saturday.

Military forces began to fire shells and rockets on the neighborhood of Khaldiyeh, a hotbed of protest, in the late evening, said activist Omar Shakir, speaking by telephone from the city. He said he heard hundreds of missiles strike the area.

The assault then spread to the Baba Amr and Bab al-Sebaa neighborhoods, with buildings collapsing on top of wailing residents. He estimated that at least 220 people were dead and more than 700 injured.

It was not possible, he said, to take the injured to hospitals because roads were blocked by security forces. Armed pro-government gangs had taken injured and dead people from the al-Amal hospital near Khaldiyeh, apparently to remove evidence of the offensive, he said.

People were being treated in makeshift field hospitals, he said, but he feared that many with head and chest injuries would die.

“Assad has lost control of this city and became mad, he became crazy to do these crimes,” he exclaimed.

Rami Abdulrahman, head of the British-based Syrian Observatory for Human Rights, said he had tallied the deaths in the city at 217 after speaking with residents. He added that Homs had been rocked by fighting between the Free Syrian Army opposition group and government forces before the assault began in the evening.

“From the 19 of March till now, this is the bloodiest day in Syria,” he declared, referring to the start of the uprising against the government of President Bashar al-Assad.

A government spokesperson declined to comment Saturday on the events of the previous night. The Syrian Arab News Agency, a state media outlet, said that footage of bodies said by the opposition to have been killed in Friday’s bombardment were in fact “the corpses of the innocent citizens kidnapped, tortured and slaughtered by the armed terrorist groups.”

The agency cited a resident of Homs who said that she recognized the corpses of two of her relatives among those shown in videos, who had been kidnapped more than two months ago. If those killed had been hit by mortars, the agency article said, “their bodies would have been turned into tiny pieces.”

The attacks in Homs and other places across Syria, an anonymous source reportedly said to the agency, had been fabricated by the opposition ahead of a crucial U.N. vote on a resolution condemning Syria.

It was not possible to independently confirm the casualty estimates or details of the attack on Homs, which is about 100 miles north of Damascus.

In Washington, President Obama issued a statement Saturday strongly condemning what he called an “unspeakable assault” by the Syrian government against the people of Homs.

“Assad must halt his campaign of killing and crimes against his own people now,” Obama said. “He must step aside and allow a democratic transition to proceed immediately.”

British Foreign Secretary William Hague also condemned the reported killings, saying they displayed Assad’s “cold-blooded cynicism” in the face of mounting international pressure for the United Nations to take action to end the bloodshed.

“The escalating violence underlines the critical importance of the U.N. Security Council adding its weight to the Arab League’s efforts to end the crisis in Syria,” Hague said.

Secretary of State Hillary Rodham Clinton, speaking to reporters at a European security conference in Munich on Saturday, said that the United States stands with Europe alongside the Arab League “in demanding an end to the bloodshed and a democratic future for Syria.”

Dima Moussa, a U.S.-based Syrian American born in Homs, said she had spoken to several city residents, who described a scene of horror.

“At least four buildings have collapsed. There are still people under the rubble. It’s the middle of the night — they can’t get to them,” said Moussa, a member of the Syrian National Council opposition group.

Moussa said the timing of the attack was significant. Opposition groups across the country had used their weekly Friday protests to commemorate the 30th anniversary of a notorious assault on an uprising in the city of Hama. At least 10,000 people were killed in that fighting, according to rights groups’ reporting at the time, and the date was highly emotional for protesters.

“I don’t think that it was a coincidence that it was the 30th anniversary of the events in Hama,” Moussa said. “The regime is saying that this is what we do, we will do it again, and they are disregarding every human rights law that’s out there, and obviously they are disregarding all the people’s demands.”

However, she predicted that the protest movement would remain resilient, saying that already in the city of Idlib, people were holding nighttime demonstrations in support of the victims in Homs.

She added that activists were reporting that there had also been a large-scale assault on Zabadani, an area near Damascus where armed opponents of the government had negotiated a cease-fire with Syrian officials more than two weeks ago. It was not possible to immediately confirm details of either operation.

The assault on Homs comes as the U.N. Security Council prepares to vote on a contentious resolution condemning the Syrian government’s response to the opposition demonstrations. Russia has strongly opposed such a move and called for a number of concessions to the proposed draft resolutions. One measure under discussion is a process of dialogue between the opposition and the Syrian government.

“It seems that the regime has read the stalling by the Russians as a license to stomp this out very quickly and then try and dictate the process of dialogue with the opposition,” said Andrew Tabler of the Washington Institute for Near East Policy.

“I don’t understand how this resolution in New York can arrest what is happening in Syria,” he added. “The idea is that the Russians would use their influence to constrain the regime, but it seems that the regime is reading it the other way.”

The violence of the crackdown against an initially largely peaceful, if now increasingly armed, nationwide protest movement has drawn international condemnation. One Western diplomat, speaking in Damascus this week, said: “The regime has no moral scruples, no shame. They subordinate all that to a sense of honor.”

In Washington on Friday night, dozens of protesters waving Syrian flags gathered outside the Syrian Embassy to protest the killings in Homs. They chanted “Down, down with Assad” and “Free, free Syria.” The Reuters news agency reported protests at Syrian embassies in Britain, Germany and Egypt, where a crowd stormed the building.

Staff writer Karen DeYoung in Washington contributed to this report.