Syria flatly rejected UN charges of crimes against humanity, even as monitors said troops killed at least 18 civilians, six in the heaviest shelling of the protest city of Homs for days.
Navi Pillay, the top human rights representative at the United Nations, said on Monday that the world body's inaction had "emboldened" Syria's government to unleash overwhelming force against its own civilians.
"The nature and scale of abuses committed by Syrian forces indicate that crimes against humanity are likely to have been committed since March 2011," she told the General Assembly.
But Syria's government rejected her accusations.
"The foreign ministry, in a message sent to the UN Human Rights Commission, categorically rejected the new allegations made by the commission," state news agency SANA said.
The ministry also accused the commission of being manipulated by "countries targeting Syria and of ignoring the terrorist crimes committed by armed groups," SANA reported.
A monitoring group said President Bashar al-Assad's forces launched one of their heaviest assaults on Homs since they began their campaign to crush rebels on February 4.
On the same day Russia and China vetoed a UN Security Council resolution on Syria for the second time in four months.
But Chinese Premier Wen Jiabao said in Beijing his country "will absolutely not protect any party, including the government in Syria."
The Syrian regime's crackdown has been centred on the central city of Homs, which has been under a relentless barrage of machinegun fire, shells, mortars and rocket-propelled grenades for 10 days.
"The shelling of the Baba Amr neighbourhood began at dawn and is the most intense in five days," Rami Abdel Rahman of the Syrian Observatory for Human Rights said.
"Two rockets are falling a minute on average," the head of the Britain-based monitoring group told AFP by telephone, citing activists on the ground.
"Six civilians died in the continuous shelling of Baba Amr neighbourhood this morning," the Observatory said later in an emailed statement.
A video uploaded to YouTube by activists showed a powerful blast striking what they said was Baba Amr, sending flames shooting into the sky and a plume of black smoke over the rebel stronghold.
Hadi Abdullah of the General Commission of the Syrian Revolution, an opposition activist group, said the shelling of Baba Amr was extremely heavy.
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"The situation is tragic. There are pregnant women, people with heart problems, diabetics and, foremost, wounded people who we cannot evacuate," he said on the phone from the beleaguered city.
"On Monday evening three activists entered the town by car transporting bread, baby milk and medicine," he said. "Their car was hit by a rocket. They all burned to death.
"We told them it was dangerous but they said, 'If we don't help the residents who will'."
Abdullah said the humanitarian situation was worsening in Homs, where vital supplies have been cut off for days, including communications, electricity, food, medicines and water.
"The urgency is to evacuate the wounded. How can we let them die in cold blood?"
"For one week, the dead have been buried in gardens, because even the cemeteries and graves are being targeted. People are crammed into shelters," he added.
Activists isolated in Homs have even reverted to the age-old practice of using carrier pigeons to communicate with each other.
"We thank Bashar for taking us back to the Middle Ages," said Omar, an activist seen standing among pigeons on a rooftop of Homs' Bab Sbaa district in a YouTube video showing them using the pigeons.
Elsewhere, another 17 people were killed Tuesday across Syria, including five soldiers who died in clashes with army deserters at Kalaat al-Madiq, in central Hama province, said the Observatory.
More than 6,000 people have been killed since Assad's forces began their brutal crackdown on protesters calling for democracy in March, according to monitoring groups. Thousands of other have fled the country, with Jordan hosting some 3,000 refugees.
The United Nations put the figure at more than 5,400 in late December, before it gave up counting the toll, citing difficulties on the ground.
The Arab League agreed Sunday on its latest initiative to stop the bloodshed, calling for the UN to approve a joint Arab-UN peacekeeping force.
Syria "categorically" rejected the proposal and Russia cast doubt on it.
US Secretary of State Hillary Clinton meanwhile expressed hope that February 24 talks to be held in Tunisia by the "Friends of Syria" coalition would bring results.
The opposition Syrian National Council was to meet Wednesday in the Qatari capital to decide whether to renew the mandate of SNC chief Burhan Ghalioun or appoint a new opposition leader, group spokeswoman Basma Kodmani told AFP.