WASHINGTON — As the city of Homs shuddered Thursday from another day of Syrian army bombardments, U.N. investigators held regime officials and military commanders "at the highest levels" responsible for "crimes against humanity and other gross human rights violations" against civilians and opposition groups.

Although it didn't name individuals, the U.N. Human Rights Council report effectively accused Syrian President Bashar Assad and his top aides of directing as a matter of state policy a "widespread and systematic" campaign of murder, torture and illegal detention aimed at smothering the 11-month-old uprising against more than four decades of Assad family rule.

Evidence gathered by the investigators shows that "orders to commit such violations originated from the policies and directives issued at the highest levels of the armed forces and the government," the report said. A sealed envelope containing the names of responsible individuals was delivered to the U.N. High Commissioner for Human Rights, it added.

Rebel groups of military deserters and civilians known collectively as the Free Syrian Army also have committed abuses, but those violations are "not comparable in scale or organization to those carried out by the state," the report said.

The report could be referred to the International Criminal Court for war crimes prosecutions, but it was unlikely to dampen Assad's crackdown. The regime insists that it is fighting Islamic extremists and not a pro-democracy movement.

The three-member panel of U.N. investigators — which was barred from entering Syria and relied on interviews with witnesses and defectors, as well as examinations of videos, photographs and satellite pictures — was unable to identify a "functioning chain of command" linking local insurgents and Free Syrian Army leaders outside of Syria. That finding raises serious questions about how the rebels could be supplied with arms and ammunition, an option that the United States and its European and Arab allies haven't ruled out.

The report was released on the eve of a "Friends of Syria" meeting in Tunisia at which senior U.S., European and Arab officials are to consider new proposals for a diplomatic resolution to the violence. The Syrian National Council, an umbrella group of opposition leaders, also is to attend.

Speaking after talks in London with key European and Arab counterparts, Secretary of State Hillary Clinton said the Syrian National Council is seen as "a credible representative" of the Syrian people. The U.N. report, however, said, "Serious rifts apparently remain within the council itself, and between the council, whose leadership largely resides abroad, and other parts of the opposition."

Assad, Clinton said, must allow deliveries of desperately needed food and medicines into rebellious cities and towns besieged by his forces.

"Our immediate focus is on increasing the pressure" on Assad, Clinton said. "We have got to find ways of getting food, medicine and other humanitarian assistance into affected areas. This takes time and it takes a lot of diplomacy."

It was unclear, however, what the United States — which has imposed sanctions on the regime and its top officials — and other countries can do to compel Assad to call a ceasefire and permit aid deliveries.