At least 51 people, including 13 rebels, died in a single day of Syrian government air strikes and shelling of opposition-controlled districts of Aleppo, activists said as international mediators were trying to salvage faltering peace negotiations in Geneva.
The Britain-based Observatory for Human Rights said most of the victims were killed by air strikes and barrel bombs dumped on eight rebel-held districts of the northern city on Wednesday.
The rest died from artillery shelling, sniper fire and in clashes between forces loyal to President Bashar Assad and rebels fighting to overthrow him.
The bombings in Aleppo, Syria's largest city, are part of a campaign by Mr Assad's forces to wrest control of districts that were seized by rebels in mid-2012.
The Observatory has been documenting Syria's conflict since its start in March 2011 through a network of activists on the ground.
It released its report on the latest Aleppo casualties today, ahead of a trilateral meeting between senior US and Russian officials and UN mediator Lakhdar Brahimi, and Syrian government and opposition representatives in Geneva.
A second round of talks started in Switzerland on Monday but the discussions quickly became mired in acrimony as government and opposition delegates hurled accusations for the bloodshed taking place back home, failing to even agree on the talks' agenda.
From the outset, the talks have been accompanied by a sharp rise in violence on the ground.
The Observatory said that at least 4,959 people have died in Syria in the three-week period since January 22, when the government and opposition delegates sat down for the first round of face-to-face meetings in Geneva.
The Observatory said in a report that the period has seen the highest death toll since the uprising against the Assad regime started nearly three years ago. More than 130,000 people have died in the conflict since then
Meanwhile Russia has presented a rival resolution to Britain and other United Nations Security Council powers after threatening to veto a Western and Arab-backed measure on sanctions against Syria.
Russia's UN ambassador Vitaly Churkin gave the text to its fellow veto-wielding permanent members of the council - the United States, Britain, France and China - at a meeting last night, a UN diplomat said. The Russian draft includes some parts of the Western and Arab motion.
Negotiations were expected to see if the drafts can be merged into a compromise text, the diplomat said. Western diplomats say their goal is to win agreement from all 15 council members on a resolution demanding immediate and unrestricted access to all areas of Syria to deliver vital humanitarian aid.
But Mr Churkin dismissed the Western and Arab text, which puts most of the blame for the humanitarian crisis on the Syrian government, as "political".
UN humanitarian chief Valerie Amos said yesterday that as violence intensified in Syria and the situation on the ground became more complicated "people are becoming more desperate".
"There are reports and allegations of systematic targeting of communities with specific religious affiliations," she said.
She told a Security Council meeting on protection of civilians that "there are 250,000 people in areas of the country which are besieged".
"They cannot leave and we cannot get aid in," said Baroness Amos, who will brief the council again today.
The governor of the central Syrian province of Homs says a ceasefire has been extended for three days as of today.
Governor Talal Barrazi says that as long as there are people who want to leave rebel-held besieged areas in Homs the truce will be extended.
There were no evacuations of Homs today. They are expected to resume tomorrow.