Syrian tanks massed outside opposition neighbourhoods in Homs on Friday as President Bashar al-Assad's forces bombarded the city for the fifth day and residents expected a major push to subdue the centre of revolt against his rule.

Assad, bolstered by Russian support, ignored appeals from world leaders to halt the assault and explosions rocked the relatively peaceful city of Aleppo in the north, fuelling fears the country would descend into all-out civil war.

Foreign ministers of the Arab League, which suspended its monitoring mission in Syria last month because of the escalating violence, will discuss a proposal to send a joint U.N.-Arab mission to Syria when they meet in Cairo on Sunday, a senior League official said.

Diplomats from Western and Arab powers have condemned Assad in strong language. But having ruled out military intervention, they were struggling to find a way to convince him to step down.

Syria blames terrorists for the 11 month uprising against Assad's rule and said on Friday they were responsible for two explosions that killed soldiers and civilians in Aleppo. Residents reported hearing explosions and gunfire and said an area around a military intelligence complex had been sealed off.

The European Union's foreign policy chief added her voice to international calls for Russia to support a United Nations resolution demanding Assad halt the crackdown. Russia has said no one should interfere in the country's affairs.

My message to my Russian colleagues is that they too need to recognize the reality of the situation on the ground and we can't go on simply allowing this to happen, Catherine Ashton said during a visit to Mexico.

Activists in Homs said shelling resumed sporadically in the morning, adding that tank reinforcements sent in the past two days raised the possibility of a major push to storm residential areas, where hundreds of thousands live.

Outgunned rebels loosely grouped under the Free Syrian Army have been preparing to counter the onslaught.

A spokesman for the rebel army said it was difficult to predict when a major attack would come but claimed Assad did not have sufficient loyalist forces to control the city.

Assad's forces are relying on shelling because they do not have the numbers. We are talking about large residential areas with a big population they would have to subdue, Major Maher al-Naimi said from Turkey.

Naimi said the Free Syrian Army will defend the city to the last drop of its blood. He declined to give details of the military balance but said that the fighting core of Assad's force was a small proportion of his forces in Homs.

Military defections have taken their toll. This is why we see large numbers of 'shabbiha' (militiamen) and security police among his forces in Homs, Naimi said.

In a message of defiance during the overnight lull, activists staged a rally against Assad in the Homs neighbourhood of al-Bayada. YouTube footage showed hundreds of youths holding hands and dancing to the tune of a songs chanted by Abdelbasset Sarout, a 22-year old footfall star turned activist.

You oppressor leave... Great Homs, Syria will be free, Sarout sang from a makeshift stage while the white and green rebel flags, dating back to before Assad's Baath Party took power in a coup five decades ago, fluttered overhead.

Activist Mohammad Hassan said a brief respite in the shelling had allowed him to leave his basement and survey the extent of the damage. There isn't one street without two buildings or more that are badly damaged from the shelling, he said by satellite phone.

We are hearing from the rebels that a big attack could occur as early as Saturday, he added.

He said artillery barrages had been directed at Baba Amro, Inshaat, Khalidiya and other districts of the city where rebels have been lying low while mounting hit-and-run guerrilla attacks on the rear of Assad's troops.

Four tanks or armoured vehicles were destroyed today on the edge of Baba Amro and some bread and medical supplies were delivered there for the first time in days by activists who crossed from Brazil Street, Hassan said.

Walid Abdallah, another activist in the city, said Alawite militiamen were playing a crucial support role for the attacking troops.

Districts such as al-Waer on the edge of the city have mostly been spared the shelling but it is now unsafe because the 'shabbiha' have begun raiding daily, he said.

Homs has become the capital of the uprising. I am afraid the regime will continue shelling it to bits and turn it into another Hama, he said, referring to the many thousands killed in the bombardment and subsequent overrunning of the city of Hama in 1982, under the rule of Assad's late father, President Hafez al-Assad.

The Free Syrian Army has been preparing for an onslaught with booby traps and what have you, Abdallah said.

A high level of social solidarity has also spread during the uprising in neighbourhoods such as Baba Amro, al-Khalidiya and beyond, and people have vowed to resist what they see as a sectarian onslaught.

HOSPITALS OVERFLOWING

Witnesses said makeshift hospitals in Homs were overflowing in besieged opposition areas with the dead and wounded from nearly a week of government bombardments and sniper fire.

Medical supplies and food were running out and, in the streets, some of the wounded had bled to death as it was too dangerous for rescuers to bring them to safety.

The Local Coordination Committees, an opposition group in Homs, put the death toll on Thursday alone as high as 110 by nightfall, though it remains impossible to verify such accounts.

This number includes three families whose bodies were dug up from under the rubble of their homes, bodies brought to field hospitals and people who died there from their wounds today, the group said in a statement sent to Reuters.

A Syrian doctor, struggling to treat the wounded at a field clinic in a mosque, delivered an emotional plea via YouTube video. Standing next to a bloody body on a table, the man, named only as Mohammed, said to the camera, and to the outside world:

We appeal to the international community to help us transport the wounded. We wait for them here to die in mosques. I appeal to the United Nations and to international humanitarian organisations to stop the rockets from being fired on us.

U.S. PONDERS AID

Concern was growing in foreign capitals over the plight of civilians.

The United States said it was considering ways to get food and medicine to them - a move that would deepen international involvement in a conflict which has wide geopolitical dimensions and has caused divisions between world powers.

I fear that the appalling brutality we are witnessing in Homs, with heavy weapons firing into civilian neighbourhoods, is a grim harbinger of things to come, U.N. chief Ban Ki-Moon said after briefing the Security Council in New York on Wednesday.

Neighbouring Turkey, which once saw Assad as an ally but now wants him out, has said it can no longer stand by and watch. It wants to host an international meeting to agree ways to end the killing and provide aid.

In Moscow, Russian foreign ministry spokesman Alexander Lukashevich reiterated that Kremlin view that though the bloodshed was regrettable, a solution was a matter for Syria.

There is an internal conflict, the word revolution is not being used - it is a not a revolutionary situation, believe me, he said.

Russia and China, which let the United Nations support the air campaign that helped oust Muammar Gaddafi in Libya, provoked strong condemnation from the United States, European powers and Arab governments when they vetoed a resolution in the Security Council last week that called on Assad to step down.

(Editing by Michael Roddy and Philippa Fletcher)