More than 3,500 people have died in months of anti-government protests in Syria, according to the UN.
A UN spokeswoman blamed "the brutal crackdown on dissent" for the figure, which was based on sources on the ground.
Last week the Arab League said Syria had agreed to pull its forces off the street in a peace deal.
But since then, the UN says more than 60 people have been reported killed - many in the central city of Homs.
'We will keep protesting'
"Syrian troops continue to use tanks and heavy weaponry to attack residential areas in the city of Homs," said Ravina Shamdasani, spokeswoman for the Office of the UN High Commissioner for Human Rights (OHCHR).
Opposition activists say security forces have been mounting a heavy offensive on the city over the past few days, particularly on the contested Baba Amr district.
The Local Co-ordination Committees, a network of Syrian activists, said that 11 people had been killed in clashes across the country on Tuesday, including four in Homs. One of those killed in Homs was reported to be a 16-year-old girl.
They say troops are going house to house to make arrests, although many residents are reported to have fled.
Ms Shamdasani described the situation in Baba Amr as "particularly appalling," adding that according to information the OHCHR has received, the area has remained under siege for seven days, with residents deprived of food, water and medical supplies.
On Monday the main Syrian opposition group, the Syrian National Council, said Homs was a "disaster area" and urged international protection for civilians.
"They think they can control Baba Amr like they did other areas but they are wrong, we are not afraid of them," said Homs-based activist Salim al-Homsi. "We will keep protesting," he told the AP news agency.
Many defectors from the armed forces have based themselves in Homs. A video posted on YouTube on Monday appeared to show a small group of defectors driving through Baba Amr over the weekend with automatic rifles and RPGs.
"We are here to protect the protesters," said a soldier who identified himself as a member of the al-Farouk brigade. "We will teach them a hard lesson," he added, referring to the military offensive.
Thousands in detention
Protests against President Bashar al-Assad's government began in March and have continued across Syria. Demonstrators claim the government has responded to peaceful protests with violence.
The authorities say the army and security forces are engaged in fighting armed gangs, and that at least 1,100 members of the security forces have been killed.
The claims are impossible to verify as the Syrian government has severely restricted access for foreign journalists.
The government said last weekend that it had released political prisoners as a first step to implementing the Arab League proposals.
However, Ms Shamdasani said that despite the release, "thousands continue to remain in detention and dozens continue to be arbitrarily arrested every day".
The Arab League has announced that it will hold an emergency meeting on 12 November to discuss "the continuation of violence and because the Syrian government did not implement its commitments in the Arab plan".
On Tuesday, the opposition Syrian National Council urged the Arab League "to take a strong and effective position against the Syrian regime commensurate with the dangerous development of the situation in Syria, especially in... Homs", AFP news agency reported.
British Foreign Secretary William Hague said it was "deplorable" that despite the Arab League deal, the Syrian government had "escalated the repression and many more people have died as a result".