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 proposals for a peace deal involving the opposition.
The UN says since then more than 60 people have been reported killed - many in the central city of Homs.
"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.
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.
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 peace deal.
However, Ms Shamdasani said that despite the release, "thousands continue to remain in detention and dozens continue to be arbitrarily arrested every day".