A UN human rights probe found on Thursday Syria had "manifestly failed" to protect its people as a defiant regime brushed off an outcry over the killing of two journalists, saying they had entered illegally and at their own risk.

Activists spoke of "terrifying explosions" in Homs as encircling regime troops pounded rebel areas for a 20th straight day, after US reporter Marie Colvin and French photojournalist Remi Ochlik were killed in the central city on Wednesday.

The international investigators said in their report that they had submitted a list of Syrian military and political officials suspected of crimes against humanity to the UN's top human rights official.

"The commission has deposited with the High Commissioner (for Human Rights, Navi Pillay) a comprehensive database containing all evidence collected," the international commission of inquiry said.

"Consistent with its mandate, the commission endeavoured, where possible, to identify those responsible with a view to ensuring that perpetrators of violations, including those that may constitute crimes against humanity, are held accountable," added the inquiry, commissioned by the UN Human Rights Council.

The commission documented a widespread and systematic pattern of gross violations committed by Syrian forces, "in conditions of impunity," since March 2011 when the uprising against President Bashar al-Assad's regime erupted.

The report said the Syrian government had "manifestly failed" to protect its people, but also said that it had found instances of gross abuses committed by rebel fighters, many of whom are mutinous soldiers.

The commission recommended the initiation of an inclusive political dialogue, bringing together the government and opposition groups.

Both sides should "negotiate an end to the violence, to ensure respect for human rights and to address the legitimate demands of the Syrian people," it said.

According to the British-based Syrian Observatory for Human Rights, more than 7,600 have been killed in the 11 months since the uprising began.

Nine people, including five soldiers, were killed on Thursday, the Observatory and state media said.

The dawn bombardment of Homs -- Syria's third-largest city -- centred on Baba Amr neighbourhood, where the two journalists were killed, a human rights watchdog said.

"Baba Amr, as well as parts of Inshaat, have been shelled since 7:00 am (0500 GMT), while mortar rounds slammed into the Khaldiyeh neighbourhood," said the watchdog's Rami Abdel Rahman.

Activist Hadi Abdullah told AFP from inside the city: "We hear terrifying explosions."

He said the world outcry over the deaths of the journalists and 24 Syrian civilians in Homs on Wednesday appeared only to have strengthened the regime's determination to eliminate all opposition in the city.

"The more the condemnations pile on, the heavier the bombing becomes," he said.

Abdullah said there was evidence that the makeshift media centre where the journalists were killed and two others wounded was deliberately targeted by regime forces.

"We are sure that the centre was targeted, because 11 rockets struck in and around it," he said.

"The regime forces intercepted a transmission signal."

The Syrian government made no denial its forces had fired the lethal rounds.

"We reject statements holding Syria responsible for the deaths of journalists who sneaked into its territory at their own risk," said a foreign ministry statement read out on state television.

The ministry urged journalists to "respect laws of journalistic work in Syria and avoid breaking the law by entering the country illegally to reach trouble-hit areas that are unsafe."

Rupert Murdoch, who owns The Sunday Times for which Colvin worked, said one of the paper's photojournalists, Paul Conroy, was also wounded.

French newspaper Le Figaro said it was trying to contact one of its reporters, Edith Bouvier, amid concerns she needs surgery for leg wounds suffered when the press centre was hit.

Meanwhile, a US official said Arab and Western powers would challenge the Syrian government to accept a proposal to allow in humanitarian aid at a "Friends of Syria" meeting in Tunis on Friday.

The move comes two days after the International Committee of the Red Cross called for a daily truce of two hours in Syria so it can deliver vital aid to afflicted areas.

But Russia said it and China, which vetoed two UN resolutions over the crackdown, "reaffirmed their joint position" of "excluding foreign intervention in Syrian affairs" and their support for talks with the regime.

And Iran's supreme leader Ayatollah Ali Khamenei said Assad's regime will not fall and that Tehran "supports the Syrian government and will oppose those who act against Syria."