Syrian forces have launched a massive bombardment of rebel districts of Homs, defying a world outcry over the killing of two Western journalists and a citizen reporter who were among the few to bear witness of the civilian toll.
Activists spoke of "terrifying explosions" on Thursday, the 20th straight day of the pounding.
The UN Human Rights Council said it had a list of Syrian officials suspected of crimes against humanity after an inquiry found that the government had "manifestly failed" in its duty to protect its own people.
But a defiant foreign ministry rejected all responsibility for the deaths of veteran American reporter Marie Colvin and French photojournalist Remi Ochlik, insisting that they had entered the country illegally and at their own risk.
The dawn bombardment of Homs - Syria's third-largest city - centred on the 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 7am (1600 AEDT), while mortar rounds slammed into the Khaldiyeh neighbourhood," the head of the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.
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 foreign 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."
French newspaper Le Figaro said one of its reporters, Edith Bouvier, was wounded in the legs in the shelling of the press centre.
Rupert Murdoch, who owns The Sunday Times for which Colvin worked, said one of the paper's photojournalists, Paul Conroy, was also injured.
Syrian citizen journalist Rami al-Sayyed, who provided live footage on the internet from Baba Amr, was also killed late on Tuesday when a rocket hit a car in which he was travelling.
Washington accused Damascus of "shameless brutality" in its bombardment of the press centre while Paris held the regime responsible.
On the eve of an international conference in Tunis dubbed the Friends of Syria, which is to gather Western and Arab governments although probably not Damascus allies China and Russia, a UN probe delivered a withering report on the regime's human rights record.
"The human rights situation in the Syrian Arab Republic has deteriorated significantly since November 2011, causing further suffering to the Syrian people," wrote the international commission of inquiry after conducting 136 new interviews since its previous report in November.
The UN Human Rights Council said it had list of political leaders and military officers suspected of "crimes against humanity".
It said investigators had left a sealed list of senior figures with the UN human rights commissioner.
International attention ahead of the Tunis conference focused on getting aid to civilians in besieged protest cities such as Homs.
The opposition Syrian National Council (SNC) held talks in Geneva with the International Committee of the Red Cross on its calls for a daily two-hour truce to allow relief supplies to be delivered.
The SNC, Syria's most representative opposition umbrella group, has called on the international community to set up "safe havens" and urged Damascus ally Moscow to force the regime to allow access for aid convoys.
Russia has given its backing to the proposal for a daily truce but Deputy Foreign Minister Gennady Gatilov said it was not backing a French proposal for aid corridors as they would require support from foreign troops.
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