Investigators on Thursday gave the UN a list of Syrian officials suspected of crimes against humanity, with the US branding Bashar al-Assad's assault on his own people as "heinous and unforgivable."
As President Assad's regime brushed off outrage over the deaths of two Western journalists in Homs, activists spoke of "terrifying explosions" while his forces pounded the city for a 20th straight day.
The White House slammed the attacks as "heinous and unforgivable," as monitors said 86 people were killed across Syria on Thursday, 61 of them civilians.
"That's why we're working with a broad array of international partners to isolate and pressure Assad, to bring around a peaceful transition in that country, a transition that is inevitable and already under way," President Barack Obama's spokesman Jay Carney said.
"We continue to believe that a political resolution is the best approach," he added, noting that arming rebels or other military involvement could "lead down a dangerous and chaotic path," though Washington would "have to evaluate this as time goes on."
The latest bombardment of Homs -- Syria's third-largest city -- centred on Baba Amr neighbourhood, where US reporter Marie Colvin and French photojournalist Remi Ochlik were killed on Wednesday.
"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 Rami Abdel Rahman of the Syrian Observatory for Human Rights.
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," Abdullah said.
In Geneva, international investigators said they had submitted a list of Syrian military and political officials suspected of possible "crimes against humanity" to UN human rights chief Navi Pillay.
The UN-commissioned panel said it documented a widespread and systematic pattern of gross violations by Syrian forces, "in conditions of impunity," since March 2011 when the popular uprising against Assad's regime broke out.
It said Syria's government had "manifestly failed" to protect its people, but also said it had found instances of gross abuses committed by rebel fighters, many of them army defectors.
The commission recommended the initiation of an inclusive political dialogue, bringing together the government and opposition groups.
Diplomats said former UN secretary general Kofi Annan was the favourite to become the international envoy on the worsening Syria crisis, amid growing pressure for an initiative on the conflict.
The Syrian Observatory, a Britain-based monitoring group, said more than 7,600 people have been killed in the 11 months since the revolt erupted.
French President Nicholas Sarkozy said the killing of the two journalists amounted to "murder" and that "those who did this will have to account for it."
Abdullah said there was evidence regime forces deliberately targeted the makeshift media centre where Colvin and Ochlik were killed and two others wounded.
"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."
"We reject statements holding Syria responsible for the deaths of journalists who sneaked into its territory at their own risk," said the foreign ministry.
Efforts were under way to evacuate the two wounded journalists -- Edith Bouvier, a reporter for French daily Le Figaro, and Sunday Times photographer Paul Conroy.
Britain said on Thursday Conroy was "on his way out" of the besieged city, while Bouvier appeared in a video posted online asking to be evacuated quickly, saying she needed urgent medical attention.
Syrian Information Minister Adnan Mahmud said later that Homs's governor had been instructed to "exert every effort possible to evacuate the journalists."
US Secretary of State Hillary Clinton said the Syrian National Council was a "credible representative" of the country's opposition, ahead of a meeting in Tunisia of the "Friends of Syria" group on Friday.
The meeting comes 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.
Beijing said it would not attend the gathering, in line with a decision by Moscow.
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."
Internet-based activists called for a day of demonstrations on Friday in support of Baba Amr.
"We will rise up for you Baba Amr," said a statement posted on the Facebook page of The Syrian Revolution 2011.
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