Syrian troops trying to quash three months of protests are committing "alleged breaches of the most fundamental rights", says a UN report.
The use of live ammunition against mostly unarmed civilians has killed around 1,100 people, says the report.
It also documents arrests on a massive scale. Investigators believe as many as 10,000 people have been detained.
Investigators used evidence from rights groups and people who had fled Syria, but were denied entry to the country.
President Bashar al-Assad is facing the gravest threat to his family's 40-year ruling dynasty, as unrest that first erupted in the south of the country has now engulfed the north - near the border with Turkey - and is threatening to spread eastwards towards its border with Iraq.
As anti-regime demonstrations continued in many parts of the country, the government on Wednesday mobilised its own demonstration of popular support as thousands of people turned out in a Damascus suburb to wave a giant Syrian flag 2.4 km (1.4 miles) long.
Laying siege to towns
UN High Commissioner for Human Rights Navi Pillay has repeatedly appealed to Syria to let in a humanitarian team make a proper assessment, but the Syrian authorities have refused.
But despite being denied access by the Syrian government, the new UN report documents reliable evidence of widespread violations, says the BBC's Imogen Foulkes in Geneva.
"The most egregious reports concern the use of live ammunition against unarmed civilians, including from snipers positioned on rooftops of public buildings and the deployment of tanks in areas densely populated by civilians," says the report.
It cites "the excessive use of force in quelling demonstrators, arbitrary detentions, summary executions, torture".
The authorities appear to have denied civilians the right to food and medical care by laying siege to towns and preventing supply deliveries, it adds.
The preliminary report was commissioned by the UN human rights council in April, adds our correspondent. A more definitive account will have to wait until the Syrian government allows UN investigators in to see the situation for themselves.
Tanks in towns
Meanwhile, Syria has called on the people of Jisr al-Shughour to return, three days after an army attack restored government control there.
More than 8,000 Syrians have fled from the north-western town into Turkey in the past week to escape military operations, which the government says are aimed at tackling "terrorist organisations".
Officials said the city of around 100,000 people - which has been the focus of large anti-regime demonstrations - was returning to normal, but that army units were still pursuing "militants" through the hills around the town.
Damascus says some 120 security personnel were killed by "armed gangs" in the town on 6 June, and on Wednesday state TV said a "mass grave" containing three bodies had been found.
At the same time, troops and tanks are reported to be massing for an attack on Maarat al-Numan, 25 miles (40km) south-east of Jisr al-Shughour. One eye-witness told BBC Arabic there were "no more than 7,000 people" left in the city, which has a population of some 90,000.
Syrian state media have reported attacks on government buildings and security headquarters in the town, which straddles the main highway between Syria's two biggest cities, Damascus and Aleppo.
Meanwhile, Jordan said it had reopened a border crossing 10km from the southern Syrian city of Deraa, where the current unrest started in March.
Although it has not yet responded to the UN report, Syria has expressed "surprise" at Arab League Chief Amr Moussa's position that the situation there was "dangerous and worrying".
Syrian rights groups put the overall death toll in Syria at 1,297 civilians and 340 security force members.
Refugees arriving in Turkey describe the regime's operation in the northern mountains as a "scorched-earth" campaign, and Syrian soldiers who deserted have said they were forced to commit atrocities.
Syria has prevented foreign journalists, including those from the BBC, from entering the country, making it difficult to independently verify reports from there.
European powers are campaigning for a draft UN Security Council resolution condemning the crackdown, but they face opposition from Russia and China, both of whom wield veto powers and object to UN action against President Assad.