The death toll, 400 higher than earlier UN estimates, was based on "reliable sources on the ground, said UN High Commissioner for Human Rights Navi Pillay, who released the data.
At least 2,600 people have been killed in Syria since pro-democracy protests broke out in March and President Bashar al-Assad sent in troops to crush the unrest, the United Nations said on Monday.
The death toll, 400 higher than earlier UN estimates, was based on "reliable sources on the ground, said UN High Commissioner for Human Rights Navi Pillay, who released the data.
The figures were almost twice the size of the Syrian government's estimate.
Bouthaina Shaaban, one of Assad's advisers, earlier on Monday said about 1,400 people had died -- half of them police officers and half opposition activists.
Syria blames armed groups and "terrorists" for the violence and argues the security forces are defending public order.
"With regard to Syria, let me note that, according to reliable sources on the ground, the number of those killed since the onset of the unrest in mid-March 2011 in that country, has now reached at least 2,600," Pillay told the 47-member UN Human Rights Council.
She did not identify the sources. Syria's government has barred Pillay's investigation team, and foreign journalists from entering the country.
Syria had also repeatedly blocked UN efforts to get human rights monitors into the country, UN humanitarian affairs chief Valerie Amos told reporters in Abu Dhabi on Sunday.
Video and mobile phone images emerging from the country during the six months of arrest have appeared to show tanks and soldiers firing on unarmed protesters.
The UN Security Council has so far failed to agree on a resolution that would impose sanctions on Syria over the violence, largely due to resistance from Russia and China.
There has been no hint in the West of any appetite for military action along the lines of the NATO bombing that helped topple Libya's Muammar Gaddafi.
Syria has three times Libya's population and has complicated ties with neighbours on the faultlines of mid-East conflicts.
After talks in Damascus with Assad, Arab League chief Nabil Elaraby said they had agreed on a series of measures to help end the violence that he would present to member states.
The European Union and the United States have already imposed their own sanctions on Syria and are considering toughening them.
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