The UN has accused the Syrian regime of "crimes against humanity" – including the use of snipers against small children – and has drawn up a list of senior officials who should face investigation, reportedly including President Bashar al-Assad.

The UN report was delivered as two journalists injured in the attack that killed Sunday Times reporter Marie Colvin and French photographer Rémi Ochlik issued dramatic appeals to be evacuated from the besieged city of Homs, where they are trapped.

A video of Edith Bouvier, a reporter for Le Figaro who suffered serious leg injuries, was released by activists in the city who say she is too badly wounded to be moved without an ambulance and guarantee of safe passage. In a second video released shortly after, Sunday Times photographer Paul Conroy, who was also injured in the attack, made a similar appeal for evacuation.

Western officials urged Damascus to give immediate humanitarian access to trapped civilian populations in Homs and elsewhere, including the evacuation of the western journalists, but said the lack of a security council mandate meant they were powerless to provide assistance without the regime's permission.

The UN report found evidence that "army snipers and Shabbiha gunmen [from pro-Assad militias] posted at strategic points terrorised the population, targeting and killing small children, women and other unarmed civilians. Fragmentation mortar bombs were also fired into densely populated neighbourhoods."

It said: "Security agencies continued to systematically arrest wounded patients in state hospitals and to interrogate them, often using torture, about their supposed participation in opposition demonstrations or armed activities."

The list of Syrian regime officials claimed to be involved in the crackdown will remain sealed until the alleged crimes can be investigated by an international human rights court. Such an investigation has so far been blocked by Russian and Chinese UN security council vetoes of concerted international action against the Damascus regime.

One commissioner who helped draw up the UN report, Yakin Ertürk, said: "All the crimes we listed came from several consistent witness accounts and showed systematic abuses."

She said the list of the named top officials believed to be involved had been kept sealed because "we are not a court. We could not investigate and sentence like a court.

"So it has been deposited with the UN high commissioner for human rights. When and if these incidents are investigated by a court, it will be made available and provide an input into the investigation."

The UN inquiry said it found "a reliable body of evidence" implicating "commanding officers and officials at the highest levels of government" in the commission of "crimes against humanity and other gross human rights violations". Although no names were released, Assad was reported to be top of the list.

The report also says rebel groups, known collectively as the Free Syrian Army, have committed torture and extra-judicial executions, but argues those violations are in no way "comparable in scale and organisation" to the abuses being carried out by the Assad regime, which have led to thousands of deaths.

"I am appalled by the evidence that young children are being targeted by snipers, and that security forces continue to arrest and torture wounded patients in state hospitals," said Alistair Burt, the Foreign Office minister for the Middle East.

"I am also very concerned at evidence of abuses by the Free Syrian Army, though the report makes clear these are on a far smaller scale than the widespread and systematic violations by the Syrian authorities. I call on all Syrians to respect human rights standards, end the violence immediately and ensure neutral and impartial access for humanitarian organisations to deliver desperately needed supplies and medical care. "

Speaking in London, on the sidelines of a conference on Somalia, the French foreign minister, Alain Juppé, warned that the Assad regime would be held accountable for its crimes "one day or another". He said: "With every passing day it gets more revolting, scandalous and shameful. The regime is massacring its people."

The minister said he had received reports that the Syrian government had ordered the governor of Homs to lift the siege on the rebel-held parts of the city, but said he could not confirm the news. He said a meeting of the Friends of Syria group in Tunis would demand a ceasefire and for humanitarian access to be allowed to besieged civilians, but he warned the international community could not force its way in.

"There is no military option at the moment on the table," said Juppé. "We are hugely frustrated. I can understand the sense of impotence. The dead are piling up. I can't say it's anything but a very deep source of anguish for me. We are doing everything we can, but we can't break the rules and act without the approval of the UN security council."

UN secretary general Ban Ki-moon and Arab League chief Nabil Elaraby on Thursday yesterday appointed Ban's predecessor Kofi Annan as joint special envoy on the Syrian crisis.

Annan "will provide good offices aimed at bringing an end to all violence and human rights violations, and promoting a peaceful solution to the Syrian crisis," a UN statement said.

Annan will also "facilitate a peaceful Syrian-led and inclusive political solution that meets the democratic aspirations of the Syrian people through a comprehensive political dialogue between the Syrian government and the whole spectrum of the Syrian opposition."

Syrian forces continued their onslaught against opposition strongholds across the country, with heavy artillery barrages against Baba Amr, the district of Homs where Colvin and Ochlik were killed.

Opposition activist Omar Shaker told the Associated Press that food, water and medical supplies were running dangerously low. "Every minute counts. People will soon start to collapse from lack of sleep and shortages in food," he said.

In a separate incident, opposition activists reported that government forces had lined up and shot dead 13 men and boys from one extended family in the village of Kfartoun in Hama province.

Chinese and Russian vetoes have complicated the international response to the humanitarian disaster unfolding in Homs and other Syrian cities in the line of fire as Assad's forces seek to destroy rebel enclaves.

Western and Arab leaders will meet in Tunis on Friday, without Russian or Chinese participation, in an attempt to unify the opposition to the regime, increase pressure for a ceasefire and prepare humanitarian relief.

The latest developments come amid strong indications that UK and US officials are working behind the scenes to attempt to unify Syria's fractured opposition.

Earlier this week the International Committee of the Red Cross called for temporary ceasefires so it could reach those trapped and wounded in the worst-affected areas.

The UN panel was denied entry to Syria by the government, which accused it of ignoring official information and exceeding its mandate.

The panel instead gathered much of its information from sources outside the country, including human rights activists and Syrian army defectors.

The report claims the ruling Ba'ath party's national security bureau was responsible for translating government policies into military operations that led to the systematic arrest or killing of civilians.

It says the four main intelligence and security agencies reporting directly to Assad – military intelligence, air force intelligence, the general intelligence directorate and the political security directorate – "were at the heart of almost all operations".

The report details how businessmen helped hire and arm informal pro-government militias known as the Shabbiha.

"In a number of operations, the commission documented how Shabbiha members were strategically employed to commit crimes against humanity and other gross violations," it said.

The report also identifies 38 detention centres "for which the commission documented cases of torture and ill-treatment since March 2011".

UN list

A panel of United Nations investigators has accused regime officials "at the highest level" of human rights violations which could subject them to prosecution.

The UN report accuses the regime of systemic attacks on the political opposition, human rights defenders and the media. It also alleges there have been widespread patterns of arbitrary arrests, disappearances and abductions.

The names on the UN list are as yet confidential but they are likely to include the following leading members of Syria's formidable security apparatus, who are alleged to have played prominent roles in the crackdown.

Major General Jumah Al-Ahmad

The commander of Syria's special forces and one of the most influential figures in the country's military. His unit is alleged to have played a key role disrupting rights protests with lethal force across the country.

Colonel Lu'ai Al-Ali

Head of Syrian military intelligence in Dera'a. The south-western city near the border with Jordan was the birthplace of the Syrian uprising last March. Violence started when troops opened fire on demonstrators who had gathered to protest against the detention of children who had been accused of writing graffiti on town walls. Dera'a remained a hub of dissent and regime-led violence throughout last summer.

Lt General Ali Abdullah Ayyub

The deputy chief of general staff (personnel and manpower), who is the officer primarily responsible for moving military forces around Syria. He is seen as a logistical key to the crackdown.

Lt General Jasim Al-Furay

The chief of general staff and one of the most trusted advisers to President Bashar al-Assad. He has supervisory oversight across all of Syria's military operations and is a key strategist and tactician.

General Aous Aslan

The head of a battalion in the Republican Guard. He is also a key adviser to Assad and to his brother, Maher al-Assad, who directs the fourth division of the Syrian Army, the unit that has been at the frontline of most of the country's flashpoint areas, particularly Dera'a, Homs and Hama.