Syria in full-scale 'civil war' - UN peacekeeping chief

United Nations observers overseeing Mr Annan's ceasefire deal were turned back from Haffeh earlier by angry crowds throwing stones and metal rods. As they left, three of their vehicles were fired on, although the monitors were unhurt.

The monitors have been trying since Thursday to visit Haffeh where activists say the army is battling hundreds of rebels and the United States has warned of another potential massacre.

Activists say Syria's army and pro-Assad militia have committed two mass killings in the last three weeks, in the Houla region and the farming hamlet of Mazraat al-Qubeir. Syrian authorities blamed the killings on "terrorists".

The killings have hardened sectarian divisions between the mostly Sunni Muslim rebels and supporters of Mr Assad - from the country's Alawite minority - in a conflict whose dynamics are also shifting as rebels become increasingly well-armed.

At least 51 civilians were killed across Syria today, the Syrian Observatory for Human Rights reported, including 13 who were killed by mortar bombs fired by security forces on a protest in the eastern city of Deir al-Zor.

Twelve soldiers also died in clashes with rebels, the body said. Syria TV reported that "terrorist groups" had seized two buses in Homs province and kidnapped all the passengers. It gave no other details.

The International Committee of the Red Cross, the only international agency to deploy aid workers in Syria, said today the situation is worsening in several parts of the country simultaneously.

"Our priority is to be able to reach as many people in as many fighting-stricken areas in the shortest delay possible. However, due to the increasingly deteriorating situation, we are unable to answer all the needs at the same time," spokesman Hicham Hassan said.

He said hundreds of people had fled this week from parts of the city of Homs, which activists say has come under the heaviest bombardment since Mr Annan's tattered ceasefire was supposed to come into force on April 12th.

Mr Annan's spokesman said he hoped to convene a meeting of an international contact group on Syria soon, saying he was encouraged by broad support for the idea.

But he gave no date for the meeting and said no venue or list of participants has yet been set.

The United States and some Western allies have resisted proposals for Iran to take part, accusing Tehran of "stage-managing" Mr Assad's crackdown on the uprising.

US secretary of state Hillary Clinton said Washington was worried Russia may be sending Syria attack helicopters and views Moscow's argument that its arms transfers to Syria are unrelated to the conflict there as "patently untrue".