LONDON — A United Nations panel concluded Thursday that “gross human rights violations” had been ordered by the Syrian authorities as a matter of state policy, amounting to crimes against humanity.

The panel of three investigators, led by Paulo Pinheiro, a Brazilian professor, said the orders had come from “the highest levels of armed forces and the government,” but did not release the names of the officials it had identified as bearing responsibility. Instead, the panel delivered the names in a sealed envelope to the United Nations’ top human rights official.

The 72-page report said that the insurgent Free Syrian Army, made up of defectors from forces loyal to President Bashar al-Assad, had also committed abuses, but that those were “not comparable in scale and organization to those carried out by the state.”

The report was released on the eve of a major international gathering in Tunisia to discuss ways to intensify diplomatic and economic pressure on Syria’s government to halt the violence. Representatives from dozens of countries are not expected to discuss military options at the meeting, even for delivering aid, but they will demand a cease- fire that would allow supplies into areas now under assault. Although there have been calls to arm the opposition, that is not on the agenda.

Syrian security forces bombarded areas of Homs, a city in central Syria, for a 20th successive day on Thursday, despite a growing outcry.

“No words can describe the damage in the streets,” said an activist in the Baba Amr neighborhood who used a nom de guerre, Al-Jid. “The snipers are spread all around. The shelling for the past two days was the worst.”

At least four people died in the city on Thursday, and activists say the toll in recent weeks has been in the hundreds; there have been frequent reports of desperate shortages of medicine and food as the siege has continued.

The Local Coordination Committees, an opposition group, said that more than two dozen people had also been killed in the city of Hama, but did not elaborate on the circumstances of the deaths. Reuters, citing activists in Hama, said that government security forces had lined up and fatally shot 13 men and boys from one extended family in the village of Kfartoun in Hama Province. Neither the death tolls, nor the report of the massacre, could be independently confirmed.

Activist groups reported dozens of people killed in heavy fighting in Idlib Province as well.

Secretary of State Hillary Rodham Clinton and several foreign ministers of European and Arab countries met in London on Thursday ahead of the international “Friends of Syria” meeting in Tunis. They discussed urgent efforts to provide humanitarian assistance, especially medical supplies to Homs and other battered cities. A senior State Department official traveling with Mrs. Clinton said that countries were ready to provide considerable assistance “within days” through United Nations relief agencies, but that “the real challenge is the access issue.”

In a press conference Thursday, Mrs. Clinton moved the United States a step closer to recognizing the Syrian National Council, an exile opposition group, though a formal recognition is not expected in Tunis. She also said later, discussing growing pressure on Mr. Assad: “There will be increasingly capable opposition forces. They will — from somewhere, somehow — find the means to defend themselves as well as begin offensive measures. And the pressure will build on countries like Russia and China because world opinion is not going to stand idly by.”

Both countries recently vetoed a United Nations Security Council resolution supporting a plan to end the bloodshed. Russia has said it will not attend the Tunisia meeting and news reports on Thursday said China had not committed, blunting the gathering’s chances of securing strong action against Mr. Assad’s government.

On Thursday, the former United Nations secretary general, Kofi Annan, was appointed as the joint United Nations-Arab League envoy on the Syrian crisis.

The United Nations report released Thursday argued that Syria was now “on the brink” of civil war and said “the continuation of the crisis carries the risk of radicalizing the population, deepening inter-communal tensions and eroding the fabric of society.”

“The commission received credible and consistent evidence,” the investigators said, “identifying high- and mid-ranking members of the armed forces who 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.”

The investigators said the report was based on 369 interviews with victims, witnesses, defectors, other people with “inside knowledge” of the situation in Syria and some government supporters. They also examined photographs, video recordings and satellite imagery to corroborate some witness accounts of the unrelenting crackdown on those who have been protesting Mr. Assad’s leadership for months.

The investigators said they had not been allowed to enter Syria to conduct inquiries, but did many interviews by phone and spoke in person with people who had fled the country.

There was no immediate response by the Syrian authorities. Accusations of human rights abuses made last November were rejected as false by the Syrian diplomatic mission in Geneva in a statement on Jan. 23 that said “armed terrorist groups,” not government forces, were responsible.

Among the scores of people who activist groups reported were killed by rockets and bombs through the day on Wednesday in Homs were Western journalists, the veteran American war correspondent Marie Colvin, who had been working for The Sunday Times of London, and a young French photographer, Rémi Ochlik. The two had been working in a makeshift media center that was destroyed in the assault. Activists said that the center had been targeted and that Syrian reconnaissance aircraft had probably spotted satellite transmitters on the roof of the building where it was located. The government denied targeting journalists.

The killings provoked outrage, with powerful media figures and European politicians calling on Syrian forces to permit the retrieval of the bodies. At least two other journalists were wounded on Wednesday, and their newspapers in London and Paris said they were trying to find ways to rescue them.

One of the wounded journalists, Edith Bouvier, 31, is a freelancer for the newspaper Le Figaro. Videos on YouTube showed her and Paul Conroy, a photographer who lives in Britain, appealing for help.

One of the videos also shows a Syrian man in medical scrubs and a stethoscope who seconded Ms. Bouvier’s call for immediate evacuation and said her life was in danger. “We need help to stop the bombardments and save Edith,” the man said in Arabic.

According to Syria’s official SANA news agency, the Foreign Ministry in Damascus on Thursday rejected “all statements that hold Syria accountable for the death of journalists who infiltrated Syria at their own risk without the Syrian authorities’ knowledge of their entry and whereabouts.”

Activists, civilian journalists and foreign correspondents who have sneaked into Syria have infuriated the authorities and foiled the government’s efforts to control the coverage of clashes.

President Nicolas Sarkozy of France called the journalists’ killings “murder,” and France’s foreign minister, Alain Juppé, said France had asked Syria to organize the rapid evacuation of Ms. Bouvier and other foreign reporters. “I vigorously protest the situation of journalists in Homs,” Mr. Juppé said, according to Agence France-Presse. “What’s happening in Syria is more revolting, more shameful and scandalous every day.”

Reporting was contributed by Rod Nordland from Cairo; Hwaida Saad and Neil MacFarquhar from Beirut, Lebanon; John F. Burns from London; Steven Erlanger, Maïa de la Baume and Scott Sayare from Paris; and Ellen Barry from Moscow. (Copyright 2012 The New York Times Company 02/24/12)