The United Nations has drawn up a list of the most senior officials in the Syrian regime, including, it is claimed, President Bashar al-Assad himself, who it says should be investigated for ordering "crimes against humanity" and other gross human rights violations.

The sealed report prepared by the UN-appointed independent international commission of inquiry on Syria has been handed over to the UN high commissioner for human rights.

While it accuses both parties to the conflict of torture and extra-judicial executions, it says that the opposition's rights 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.

The news of the report's delivery came as the Syrian city of Homs endured its 20th straight day of shelling, which has claimed hundreds of lives including those of the Sunday Times journalist Marie Colvin and the French photographer Rémi Ochlik.

The disclosure that the most senior figures in the regime are on the list comes a day before a Friends of Syria meeting in Tunis on Friday. Discussions are also expected on Thursday on the fringes of the Somalia conference in London, where the foreign secretary, William Hague, is expected to discuss additional measures that could be taken to isolate the Syrian regime amid growing calls to arm the rebels.

"A reliable body of evidence exists that, consistent with other verified circumstances, 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," the report states.

"The commission has deposited with the United Nations high commissioner for human rights a sealed envelope containing the names of these people, which might assist future credible investigations by competent authorities."

It doesn't say who these investigating authorities may be, but the UN's top human rights official has previously called for Syria to be referred to the international criminal court. Members of the 47-nation UN Human Rights Council are expected to hold a special meeting on Syria in Geneva next week, at which the panel's report will be formally presented.

International pressure has been building on Assad's government to halt its violent suppression of the opposition.

Speaking to the BBC, Hague outlined the measures he would like to see emerge from the meeting in Tunis.

"It is a deeply frustrating situation," said Hague, adding that the meeting would seek a "wide set of measures across a large group of nations" to place a "diplomatic and economic stranglehold" on Syria.

The latest developments also 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 that 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".

Armed opposition groups, loosely connected under the umbrella of the Free Syrian Army, also committed some gross human rights abuses, the panel said. It cited the torture and execution of soldiers or suspected pro-government militia members.

But such actions were "not comparable in scale and organisation with those carried out by the state", it added.