Navi Pillay told a closed session of the Security Council that 14,000 people are believed to have been arrested and 12,400 fled to neighbouring countries. More than 5,000 deaths did not include security forces which are estimated at more than 1,000.

At least 20 people died in clashes on December 11.

Amid the violence, local polls were held, but turnout is expected to have been very low.

Authorities said the vote had been freer than in previous years, but the opposition called for a boycott and launched a general strike.

The UN official said the protesters in Syria had remained largely peaceful since the uprising erupted in March, but that attacks against Syrian government had been increasing.

The EU has imposed 10 rounds of sanctions on the Syrian government, and the Arab League has suspended it, but the UN has not yet passed a resolution condemning Damascus.

Russia and China both vetoed a European-led draft at the UN in October, while India, South Africa and Brazil have also been reluctant to support action at the Security Council.