(Reuters) - Western and Arab nations will demand that Syrian forces implement an immediate ceasefire to allow relief supplies to reach desperate civilians in cities such as Homs which has endured a 20-day bombardment when they meet in Tunis on Friday.

Piling pressure on President Bashar al-Assad, U.N. investigators accused his security apparatus of crimes against humanity as world outrage mounted over violence that has cost thousands of lives during an almost year-long popular uprising against his 11-year rule.

The Friends of Syria meeting will call on Syrian forces to stop firing to give international aid groups access to areas worst hit by the violence which are running out of medicine and food, according to a draft declaration obtained by Reuters.

The draft also recognized 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.

About 70 nations, including the United States, Turkey, and European and Arab countries that want Assad to step down, will take part in the talks, but Russia and China, which have jointly vetoed two U.N. Security Council resolutions on Syria, say they will stay away.

The Syrian National Council is allied with the Free Syrian Army (FSA), made up mostly of army deserters fighting security forces that have sought to crush protests against Assad, whose minority Alawite sect dominates Sunni-majority Syria.

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. Two people were killed by troops at a checkpoint inside the city.

CRIMES AGAINST HUMANITY

Such accounts are hard to verify due to Syrian government restrictions on independent journalists.

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 neighborhood.

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 neighboring 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.

HUMANITARIAN ACCESS

The army is blocking medical supplies to parts of Homs and electricity is cut off 15 hours a day, activists say.

The International Committee of the Red Cross has been trying to arrange daily two-hour ceasefires, so far without success.

To further isolate Assad's government, the European Union will impose more sanctions on Syria next week.

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 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.

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