The top United Nations human rights official called on Syria on Thursday to halt its "assault on its own people".
United Nations High Commissioner for Human Rights Navi Pillay said more than 1100 people may have been killed and up to 10,000 detained since March.
She urged Damascus to allow a fact-finding mission into the country to investigate all allegations, including Syrian state television reports that 120 members of the security forces were killed by armed gangs.
Her statement came as international pressure mounted on the UN Security Council to condemn the regime of President Bashar al-Assad.
Syria's government is attempting to "bludgeon its population into submission" with tanks, artillery and snipers, Ms Pillay said.
Ms Pillay, a former UN war crimes judge, denounced the death of 13-year-old Hamza al-Khatib, whom activists say was tortured and killed by security forces.
Syrian authorities deny that, saying he was killed at a demonstration in which armed gangs shot at guards.
On Wednesday, Britain, France, Germany and Portugal handed the Security Council a draft resolution condemning Mr Assad's crackdown, despite the risk of a Russian veto.
Meanwhile, Turkish officials say about 2400 Syrians have fled to Turkey fearing a government assault on the northern town of Jisr al-Shughour in retaliation for the killing of members of the security forces.
Nuclear body reports Syria to Security Council
The UN nuclear watchdog is to report Syria to the Security Council over its alleged covert nuclear programme.
The International Atomic Energy Agency (IAEA) voted 17-6 to rebuke Syria over claims of an undeclared nuclear reactor, with Russia and China among those opposing the move.
The alleged nuclear structure, which Syria has maintained was a non-nuclear military site, was destroyed by Israel in 2007.