More than 7,500 people have died in Syria since security forces launched a crackdown on dissent last March, a senior United Nations official says.
Under-Secretary-General for political affairs Lynn Pascoe said there were "credible reports" the toll often exceeded 100 civilian deaths per day.
Syrian forces pounded residential areas in Homs and elsewhere, on Tuesday, killing scores, activists reported.
Meanwhile, a UK journalist hurt in Homs last week, was smuggled to safety.
Sunday Times photographer Paul Conroy is reported to be safe and well in Lebanon, but the whereabouts of Frenchwoman Edith Bouvier, who has a broken leg, and two colleagues is unclear.
The four journalists left Homs together but their convoy was shelled and they were separated, said the Avaaz campaign group, which said it co-ordinated the rescue attempt.
French President Nicolas Sarkozy withdrew an earlier statement that Ms Bouvier had reached safety in Lebanon.
The UN's new estimate of the number of deaths came at a meeting of the UN Security Council.
"There are credible reports that the death toll now often exceeds 100 civilians a day, including many women and children," said Lynn Pascoe.
"The total killed so far is certainly well over 7,500 people."
The Syrian government says at least 1,345 members of the security forces have been killed combating what it calls "armed gangs and terrorists", and puts the number of civilians killed at 2,493.
In Washington, US Secretary of State Hillary Clinton said that President Bashar al-Assad fitted the definition of a war criminal.
She told a Senate hearing that an argument could be made for declaring the Syrian leader a war criminal.
Mrs Clinton added that such an action would complicate the chances of finding a solution to the violence - and tended to make it harder to persuade a leader to stand down.
An emergency meeting of the UN Human Rights Council was also held in Geneva, where commissioner Navi Pillay said atrocities against civilians were being committed.
Syria's representative to the UN, Faysal Khabbaz Hamoui, stormed out of the session, accusing countries of "inciting sectarianism and providing arms".
With no clear word on the fate of Ms Bouvier and her colleagues, campaign group Avaaz said activists had died taking part in the rescue attempt.
Executive director Ricken Patel said the rescue group had been split in two by shelling after leaving Homs, and only Mr Conroy's group had been able to move forward.
Avaaz described the three other journalists - Ms Bouvier, Javier Espinosa from Spain and Frenchman William Daniels - as "unaccounted for".
Mr Conroy was apparently able to walk across the border into Lebanon during the night, but the BBC's Jim Muir in Beirut adds that the more seriously wounded Ms Bouvier would have had to be carried on a stretcher.
Confusion remains over Ms Bouvier's location.
She and Mr Conroy were both hurt in the attack in which journalists Marie Colvin and Remi Ochlik died. There has been no word on what has happened to their bodies.
Reports on Tuesday said Homs had come under some of its heaviest bombardment yet, with the government sending in units of an elite armoured division into rebel-held districts.
The activists' network the LCC said 65 people were killed on Tuesday. Death tolls remain very hard to verify as media access across the country is tightly restricted.
'Crimes against humanity'
The emergency session of the United Nations Human Rights Council (UNHRC) discussed a confidential report delivered by a UN panel of experts that lists Syrian army officers and government officials who could be investigated for crimes against humanity.
French Foreign Minister Alain Juppe has urged the 47 nations in the council to be prepared to submit a complaint against Syria to the International Criminal Court (ICC) in The Hague.
"The task of the council is to express the disgust of the entire world at the odious crimes that the Syrian state is committing against its people," he said.
But the meeting is unlikely to bring about any change from the government in Damascus which is currently fighting for its survival, says Jim Muir.
It is more likely to put pressure on countries such as Russia and China, which have opposed any international action against Syria, he adds.