More than 2,200 people have been killed since the Syrian government's crackdown on protesters began in March, says the UN high commissioner on human rights.
Navi Pillay said the new toll included 350 deaths reported since the beginning of the Islamic holy month of Ramadan.
Later, UN Secretary General Ban Ki-moon said it was "troubling" that President Bashar al-Assad had not kept his word about halting military operations.
Several people were killed in Homs as crowds welcomed a UN humanitarian team.
On Sunday, Mr Assad insisted that his government was in no danger of falling and warned that any foreign military intervention would backfire.
Opening an emergency session of the UN Human Rights Council in Geneva on Monday, Ms Pillay said: "The gravity of ongoing violations and brutal attacks against the peaceful protesters in that country demand your continued attention."
She went on: "As of today, over 2,200 people have been killed since mass protests began in mid-March, with more than 350 people reportedly killed across Syria since the beginning of Ramadan."
"The military and security forces continue to employ excessive force, including heavy artillery, to quell peaceful demonstrations and regain control over the residents of various cities."
The meeting followed the publication of a report by UN investigators earlier this month which concluded that Syrian security forces were carrying out widespread human rights violations, which could constitute war crimes.
The 47-member council is considering a draft resolution that "deplores the continuing indiscriminate attacks on its population" and seeks an immediate stop to "all acts of violence".
The resolution also stresses the need to "urgently dispatch an independent international commission of inquiry... to investigate violations of international human rights law in Syria since July 2011".
Faysal Khabbaz Hamoui, Syria's ambassador to the UN, defended his government and dismissed the allegations as "mere lies".
"Syria has been subjected to and continues to be subjected to an unprecedented misleading campaign carried by a number of countries in order to weaken Syria and to change its political position," he said.
He described the language used in the resolution as "hateful" and urged council members not to support it. "The resolution will only cause the crisis to lengthen and will only cause more instability," he said.
The BBC's Imogen Foulkes in Geneva says that while human rights council decisions carry moral authority, the UN has no power to enforce them.
Real pressure for regime change would have to come from the UN Security Council where at least one permanent member, Russia, remains opposed, our correspondent adds.
The US ambassador to the UN, Eileen Donahoe, made it clear that the meeting was about more than simply condemning human rights violations.
"The purpose of today's session is to increase the pressure on the Assad regime, to get Assad to step down, and to allow the Syrian people to move forward," she said.
Later, the UN secretary general criticised Mr Assad for not keeping a promise that military and police operations against demonstrators had ended.
"It's troubling that he has not kept his words," Mr Ban told reporters at the UN headquarters in New York. "I sincerely hope that he heeds... all [the] international community's appeals and calls."
His comments came as activists said three people had been shot dead in the third city of Homs as crowds took to the streets after the visit of a UN team that has been granted access to assess humanitarian needs.
Inspired by the recent events in Libya, thousands gathered at the central Clock Square, chanting: "Gaddafi is gone, it's your turn Bashar."
But it was not before cars full of security personnel and militiamen drew up and opened fire, according to amateur video posted on the internet.
In one, a man can be heard saying: "The security forces are shooting directly onto the demonstrators just after the UN delegation left. Repeated shooting."
The BBC's Jim Muir in Beirut says the UN team may have been only dimly aware of what was going on.
Wherever they have gone since their mission began on Sunday, they have been mobbed by protesters trying to get the voices to the outside world, our correspondent says.
The authorities have tried to give them a sanitised view of the country, but government officials travelling with the delegation have been deeply embarrassed by what one diplomat has called "an attempted whitewash that has turned into a fiasco for the regime", he adds.