A three-member panel based its 72-page report on 369 interviews with victims, witnesses, defectors and others with "inside knowledge" of the situation in Syria, The New York Times reported.
The report, which cited torture and killing of civilians, said the nation was "on the brink" of civil war and "the continuation of the crisis carries the risk of radicalizing the population, deepening inter-communal tensions and eroding the fabric of society."
The report said "credible and consistent evidence" showed members of the armed forces had "ordered their subordinates to shoot at unarmed protesters, kill soldiers who refused to obey such orders, arrest persons without cause, mistreat detained persons and attack civilian neighborhoods with indiscriminate tanks and machine gun fire."
Evidence, the report said, "provides reasonable grounds to believe that particular individuals, including commanding officers and officials at the highest levels of government, bear responsibility for crimes against humanity and other gross human rights violations."
"Gross human rights violations were conducted pursuant to a policy of the state," the report said and the "orders to commit such violations originated from policies and directives issued at the highest levels of the armed forces and the government."
The Times said there was no immediate response from Syrian authorities.
The report, delivered to the U.N. Human Rights Council in Geneva, also said the Free Syrian Army, comprising defectors from forces loyal to President Bashar Assad, had also committed abuses, but they were "not comparable in scale and organization to those carried out by the state."
The report came as Britain and France demanded the Syrian government return the bodies of two slain journalists and treat three other injured journalists.
Meantime, world leaders were preparing to try to build pressure on Assad to end an 11-month-old crackdown on pro-democracy demonstrations, CNN reported.
The three journalists reportedly were in Homs to document the attacks by government forces when they were wounded Wednesday in shelling that killed American reporter Marie Colvin, working for a British newspaper, and French photographer Remi Ochlik.
Assad has denied targeting civilians and said military action was necessary to protect Syrians from "terrorists" and foreign fighters trying to destabilize Syria.
The French and British foreign ministries summoned Syria's ambassadors in their countries and told Syria it was expected to make arrangements to return the bodies of Colvin and Ochlik, as well as provide medical attention to the injured journalists.
In London, the Foreign Office said in a statement it demanded the "unacceptable violence" in Homs stop immediately, CNN reported.
"The Syrian authorities must implement the undertakings they had given to the Arab League, halt all violence against civilians, and start an orderly political transition" before anyone else is killed, the statement said
Syria's Information Ministry said it had no knowledge of the journalists being attacked and officials in Homs province were looking into the reports, the state-run Syrian Arab News Agency said.
While offering condolences, the Syrian Foreign Ministry denied Syria was responsible for the deaths of journalists "who infiltrated its territory on their own," Syrian state TV said.
The demands concerning the journalists came as the International Committee of the Red Cross in Damascus was reported trying to negotiate a temporary cease-fire and humanitarian aid corridor, CNN said.
"We are back talking to everybody and anybody on how to arrange a two-hour cessation of conflict," said Saleh Dabbakeh, a Red Cross communication delegate in Syria.
The negotiations concerned not only getting journalists out but also any civilians trying to flee the conflict in Homs, Dabbakeh said.
Russian Foreign Ministry spokesman Alexander Lukashevich said Wednesday Moscow was "seriously concerned" about the civilian plight in Homs.
CNN said reports indicated at least four people died in Homs Thursday, the 20th straight day of shelling in the besieged city.
At a meeting Friday in Tunisia, world leaders are expected to increase pressure on the Assad regime, the U.S. State Department said.
More than 70 countries were invited to participate in the "Friends of Syria" group, described as part of "ongoing efforts with our friends, allies, and the Syrian opposition to crystallize the next steps to halt the slaughter of the Syrian people and pursue a transition to democracy in Syria," State Department spokeswoman Victoria Nuland said.
The United Nations said it has documented 5,400 deaths as of January, when it said it was no longer able to safely gather information. Saudi Arabia says the death toll is now at least 7,000.
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