Syrian tanks pushed into a rebel stronghold in the battered city of Homs on Thursday and U.N. investigators accused President Bashar al-Assad's government of crimes against humanity.
The plight of Homs and other embattled towns will dominate Friends of Syria talks in Tunis on Friday involving the United States, European and Arab countries, Turkey and other nations demanding that Assad halt the bloodshed and relinquish power.
Russia and China, which have jointly vetoed two U.N. Security Council resolutions on Syria, say they will stay away.
The meeting will call on Syria to implement an immediate ceasefire to give aid groups access to areas worst hit by the violence, according to a draft declaration obtained by Reuters.
The draft also recognised the Syrian National Council as a legitimate representative of Syrians seeking peaceful democratic change, a phrase which appeared to fall short of full endorsement of the most prominent group opposed to Assad.
The exiled SNC is allied with the Free Syrian Army (FSA), made up of army deserters and insurgents who are resisting security forces that have sought to crush protests against Assad's 11-year rule, bolstered by his minority Alawite sect.
Syrian security forces lined up and shot dead 13 men and boys from one extended family, which has the same name as the FSA's commander Riad al-Asaad, in the village of Kfartoun in Hama province on Thursday, activists in Hama city said.
It was not immediately clear if the victims were related to Asaad, who is based in Turkey and comes from the northwestern province of Idlib.
Activists said three people were also killed in shelling of the nearby village of Soubin. The bodies of five Syrian workers who disappeared two days ago after crossing from Lebanon on their way to Hama were found on Thursday, they said.
CRIMES AGAINST HUMANITY
World outrage has swelled over the carnage in Syria, where thousands have been killed since the anti-Assad uprising flared in March, inspired by revolts against Arab autocrats elsewhere.
U.N. investigators said Syrian forces had shot and killed unarmed women and children, shelled residential areas and tortured wounded protesters in hospital under orders issued at the highest levels of the army and government.
In their report to the U.N. Human Rights Council, they called for perpetrators of such crimes against humanity to face prosecution and said they had drawn up a confidential list of names of commanders and officials alleged to be responsible.
The commission found that Free Syrian Army rebels had also committed abuses although not comparable in scale.
Syrian authorities have not commented, but they rejected the commission's previous report in November as totally false.
Rockets, shells and mortar rounds rained on the Baba Amro district, where armed insurgents are holed up with terrified civilians, for the 20th day in a row, activists said. The Sunni Muslim quarters of Inshaat and Khalidiya also came under fire.
Homs-based activist Abu Imad said tanks had entered the Jobar area in the south of Baba Amro.
Explosions are shaking the whole of Homs. God have mercy, Abdallah al-Hadi said from the city, where more than 80 people, including two Western journalists and Syrian opposition citizen journalist Rami al-Sayed, were reported killed on Wednesday.
Western diplomats said it had not yet been possible to extract the bodies of Marie Colvin, an American working for Britain's Sunday Times, and French photographer Remi Ochlik.
Two journalists wounded in the same attack - British photographer Paul Conroy and French reporter Edith Bouvier, along with French photographer William Daniels, who was unhurt - were also awaiting evacuation from the Baba Amro neighbourhood.
Bouvier, in a YouTube clip posted by activists, said she urgently needed an operation on a broken leg and appealed for a ceasefire and medical transport to neighbouring Lebanon.
The Syrian Information Ministry rejected accusations that Syria was responsible for the deaths of journalists, who infiltrated into the country on their own responsibility.
U.S. officials said the Friends of Syria group would challenge Assad to provide humanitarian access within days to civilians embroiled in the intensifying conflict.
The army is blocking medical supplies to parts of Homs and electricity is cut off 15 hours a day, activists say. Hospitals, schools, shops and government offices are closed.
The International Committee of the Red Cross has been trying to get the government and rebel forces to agree daily two-hour ceasefires. Access for aid workers will also be the focus of a planned visit to Syria by U.N. humanitarian chief Valerie Amos.
To further isolate Assad's government, the European Union will impose sanctions on seven Syrian cabinet ministers next week. It will also slap new curbs on the Syrian central bank, prohibit trade in precious metals with state institutions and ban cargo flights from Syria, a senior EU diplomat said.
The bloody siege of parts of Homs has aroused widespread international indignation, but the world has so far proved powerless to alleviate the predicament of civilians there.
Footage shot by activists in Homs shows blasted buildings, empty streets and doctors treating casualties in makeshift clinics in Baba Amro after nearly three weeks of bombardment.
Several hundred people have been killed in Homs by troops using artillery, tanks, rockets and sniper fire.
Residents fear Assad will subject the city to the same fate his late father Hafez inflicted on Hama, where many thousands were killed in the crushing of an armed Islamist revolt in 1982.
The United States, which so far has been against military intervention in Syria, has hinted that if a political solution were impossible it might have to consider other options.
Republican presidential candidates Mitt Romney and Newt Gingrich on Wednesday advocated arming Syrian rebels.
The state news agency SANA said three members of the security forces were killed and seven wounded by a bomb planted by armed terrorists near the city of Idlib. It also reported the funerals of 16 security force members killed by rebels.
Assad has called a referendum on a new constitution on Sunday, to be followed by a multi-party parliamentary election, which he says is a response to calls for reform. The plan is supported by his allies Russia and China but Western powers have dismissed it and the Syrian opposition has called for a boycott.
(Additional reporting by Dominic Evans, Erika Solomon and Mariam Karouny in Beirut, Stephanie Nebehay in Geneva, Arshad Mohammed in London and Don Durfee and Ben Blanchard in Beijing; Writing by Alistair Lyon; Editing by Mark Heinrich and Robin Pomeroy)