Syrian government forces and allied militia have committed war crimes including murder and torture in what appears to be state-directed policy, UN investigators have said.
Syrian rebels fighting to topple President Bashar al-Assad have also committed war crimes, but these "did not reach the gravity, frequency and scale" of those carried out by the army and security forces, the investigators said.
The report called for the UN Security Council to take "appropriate action" given the gravity of documented violations by all sides.
The Security Council can refer a case to the prosecutor of the International Criminal Court, a United Nations tribunal, but Russia and China are reluctant to condemn Syria.
"We have identified both parties as guilty of war crimes and of course a greater number and of bigger variety from the government side," Karen AbuZayd, a senior US investigator, told Reuters.
"What happened on the government side appears to be a policy of the state. It is not just widespread but similar large-scale complex operations, how they are carried out, the way the military and security work together," she said.
The independent investigators, led by Brazilian Paulo Pinheiro, conducted more than 1,000 interviews over the past year to produce their latest 102-page report for the UN Human Rights Council.
Many of the interviewees were Syrian refugees or defectors who have fled to neighbouring countries.
They found "reasonable grounds" to affirm that government forces and the allied shabbiha militia had committed crimes against humanity, war crimes and gross violations of international human rights and humanitarian law.
These included "unlawful killing, torture, arbitrary arrest and detention, sexual violence, indiscriminate attack, pillaging and destruction of property".
Meanwhile, an air raid killed 30 people in the rebel-held northern border town of Azaz today, a local doctor said.
Doctor Mohammad Lakhini said at a hospital in Azaz that scores of people were wounded in the air strike.
It reduced several houses in the town to rubble.
Seven Lebanese hostages being held in Azaz were also wounded, with four others still missing, a rebel commander said.
Mr Assad's forces have increasingly used helicopter gunships and warplanes against the lightly-armed insurgents.
In Damascus, a bomb exploded in the car park of a hotel used by UN monitors, but several military buildings are also in the vicinity and it was not clear what the target was.
No UN staff members were hurt in the blast. The bomb set a fuel tanker ablaze and black smoke billowed over the city. Ash and dust covered white UN vehicles parked nearby.
State media said three people were wounded in the bombing and several rebels were killed or captured in a separate gun battle with security forces in the western district of Mezze.
Deputy Foreign Minister Faisal Mekdad said the bomb blast proved "the criminal and barbaric nature of those who carry out these attacks - and their backers in Syria and abroad".
UN emergency relief coordinator Valerie Amos, on a mission to seek more access for aid deliveries, was meeting EU officials in Damascus when the bomb went off.
She was unable to reach the town of Douma, a trouble spot just north of the capital, due to bombardment.
"Waiting at checkpoint to get into Duma. Sounds of shelling. Could not enter," she tweeted. The authorities told her she had been turned back for her own safety, her spokesman said later.
Groups calling themselves The Descendants of the Prophet Brigade and the al-Habib al-Mustafa Brigade said on a Facebook page they were jointly responsible for the bombing and that the attack had killed 50 soldiers. It was impossible to verify that claim.
Last month, Mr Assad's troops successfully counter-attacked after rebels seized parts of Damascus. They are still trying to dislodge insurgents from Aleppo, Syria's biggest city.
Elsewhere, a Syrian air strike has wrecked a hospital in a rebel-held area of Aleppo, a doctor there said today, an attack that New York-based Human Rights Watch said violated international law.
At least two holes gaped in the walls of Al Shifaa Hospital and four floors were heavily damaged by Tuesday's raid.
The bloodshed has divided regional and world powers, foiling peace efforts and paralysing the UN Security Council on Syria.
Russia accused the West today of reneging on an agreement to establish a transitional government in Syria and of prolonging the bloodshed by encouraging the rebels to keep fighting to overthrow Mr Assad.
Foreign Minister Sergei Lavrov said an agreement made by world powers and the then-peace envoy Kofi Annan in Geneva on 30 June was still valid and urged the West to do more to put it into practice.
Most Western and Arab governments have called on Mr Assad to go, saying his government's violent response to initially peaceful protests give him no place in a future Syria.
Russia has opposed tougher UN sanctions against Damascus, a long-time strategic ally, but denies it is actively helping Mr Assad remain in power.
Muslim heads of state were expected to suspend Syria from the Organisation of Islamic Cooperation at a summit in Mecca today, despite the objections of Iran, Mr Assad's closest ally.
The 57-member body's rebuke is mostly symbolic, but it shows Syria's isolation in much of the Sunni-majority Islamic world.