Syrian forces killed at least 41 civilians on Saturday's eve of a referendum on a new constitution called by the regime in the face of an 11-month uprising, as the Red Cross failed to agree a deal to evacuate wounded Western journalists.

Embattled President Bashar al-Assad's forces resumed shelling the Baba Amr district of Homs after an apparent pause to allow in relief teams, more than three weeks into a deadly assault on rebels in Syria's third largest city.

They also attacked elsewhere, killing at least 41 civilians nationwide, including 19 in Homs, said the Britain-based Syrian Observatory for Human Rights.

And 16 soldiers and security force members died in explosions and clashes with rebels.

Police also fired on a demonstration of some 4,000 people in Aleppo city's Sayef al-Dawla district, at the funeral of a civilian killed on Friday.

In Homs, the International Committee of the Red Cross (ICRC) said it had been in talks to resume evacuations from Baba Amr where two wounded Western journalists are trapped along with the bodies of two others killed on Wednesday.

But the negotiations with the Syrian authorities and opposition groups ended in failure Saturday, the ICRC's Saleh Dabbakeh told AFP.

The "ICRC and Syrian Arab Red Crescent have been negotiating since this morning with both the Syrian authorities and opposition groups in Homs. The discussion has yielded no concrete result today. Unfortunately, therefore, no emergency evacuation will take place today," he said.

"The ICRC and the SARC will continue to negotiate with the authorities and opposition in an attempt to enter Baba Amr and carry out life-saving evacuations," he added.

Dabbakeh earlier confirmed that the Red Cross and the Syrian Arab Red Crescent (SARC) evacuated seven Syrians wounded in shelling on Friday, as well as 20 sick women and children. They were taken to Homs' Al-Amine hospital.

A Western diplomat confirmed that the ICRC and SARC were still negotiating on the evacuation of the two wounded Western journalists and the bodies of two others.

France also said on Friday it was intensifying diplomatic efforts to rescue the wounded journalists.

"We are pursuing our efforts more than ever to obtain a secure medical evacuation of the foreign journalists," foreign ministry spokesman Bernard Valero told AFP.

The Red Crescent said on its Facebook page that "in addition to the seven wounded, it also evacuated 20 women and children" on Friday.

Eleven ambulances and other vehicles drove into Baba Amr, but only three ambulances left with hurt Syrians, although Dabbakeh said earlier the operation would also include the journalists.

American reporter Marie Colvin and French photojournalist Remi Ochlik were killed on Wednesday when a rocket hit a makeshift media centre in Baba Amr, a rebel stronghold.

French reporter Edith Bouvier and British photographer Paul Conroy suffered leg wounds in the attack.

Assad's government accused rebels in Baba Amr of refusing to hand over Bouvier and the bodies of the journalists to rescuers.

"The concerned authorities in Homs, moved by humanitarian considerations, sent several local officials and Red Crescent ambulances to evacuate the Western journalists who entered Syria illegally," SANA state news agency quoted an official as saying.

"Despite efforts that lasted several hours, armed groups in Baba Amr refused to hand over the wounded woman (Bouvier) and the two bodies, thus endangering the life of the wounded French journalist," the official added.

With the violence showing little sign of easing, Syrians have been called to vote on Sunday on a new constitution aimed at appeasing protesters and fending off growing global pressure on Assad to step down.

But the referendum on the constitution, which could end five decades of Baath party rule, has already been dismissed by the United States as "laughable."

In the Tunisian capital on Friday, some 60 governments gathered for the first meeting of the "Friends of Syria" group expressed "strong concern" about the humanitarian situation.

The group said it would deliver humanitarian supplies immediately, if the regime ended the violence.

It also called for a "political solution" to the crisis and recognised the Syrian National Council, the main opposition coalition, as "a legitimate representative of Syrians seeking peaceful democratic change."

Syrian Information Minister Adnan Mahmud took a poetic swipe at the Friends of Syria meeting, calling it a gathering of the "friends of Washington and the enemies of Syria."

"Participants took only one decision, to continue supporting the terrorists and furnish them with weapons to attack the security and stability of Syria," he told reporters.

And China's Xinhua state news agency accused the United States and Europe of "harbouring hegemonistic ambitions" in Syria.

Beijing and Moscow, which have so far frustrated efforts to rein in Assad's regime, boycotted the Tunis meeting.

Western and most Arab governments have so far rejected the idea of a foreign mission such as the NATO-led operation that helped topple Moamer Kadhafi's regime in Libya last year.