At least 20 people were killed as Syrian tank forces battled opponents of President Bashar Al-Assad in Homs on Monday, residents said, ahead of a planned visit by Arab League monitors to verify whether he is ending a violent crackdown on unrest.

A day before observers were to have their first look at the city at the heart of a nine-month-old revolt, there was no sign of Al-Assad carrying out a plan agreed with the League to halt an offensive against protests and start talks with the opposition.

Amateur video posted to the internet by activists showed three tanks in the streets next to apartment blocks. One was firing its machinegun and another appeared to be firing mortar rounds. Gruesome video showed mangled bodies lying in pools of blood along a narrow street. Power lines had collapsed and cars were burnt and blasted, as if by mortar rounds.

An armed insurgency is increasingly eclipsing civilian protests. Now many fear Syria is sliding toward a sectarian war that pits the Sunni Muslim majority, the driving force of the protest movement, against minorities that have mostly stayed loyal to the government, particularly the Alawite sect to which Al-Assad belongs. Fighting in Homs has intensified since a double suicide bombing in Damascus on Friday that killed 44 people.

'The Baba Amr (district) (of Homs) is being exposed to fierce shelling from heavy machinegun fire, armoured vehicles and mortars,' the British-based Syrian Observatory for Human Right said in a statement.

'The violence is definitely two-sided,' said a Homs resident who named himself only as Mohammed to protect his safety. 'I've been seeing ambulances filled with wounded soldiers passing by my window in the past days. They're getting shot somehow.'

Parts of Homs are defended by the Free Syrian Army, made up of defectors from the regular armed forces, who say they have tried to establish no-go areas to protect civilians.

The Observatory documented names of those reported killed in Monday's clashes sparked by a new wave of raids and arrests that also hit districts in Syria's second city Aleppo. That business hub had been spared most of the upheaval until recently.

The initial 50 of 150 Arab League monitors were due to arrive in Syria on Monday, and some will go to Homs on Tuesday, a source at the group's headquarters in Cairo told Reuters.

Their job will be to assess whether Al-Assad is withdrawing tanks and troops from Syria's third largest city as promised.

'The element of surprise will be present,' said monitor Mohamed Salem al-Kaaby from the United Arab Emirates. 'We will inform the Syrian side the areas we will visit on the same day so that there will be no room to direct monitors or change realities on the ground by either side.”

The mission's objective is to confirm that the Syrian government is executing the Arab League initiative by withdrawing the military from cities, releasing prisoners and allowing Arab and allowing international media to visit.      Syria has barred most foreign journalists since the revolt began, making it hard to verify reports of events on the ground.

Other residents in Homs said they were running low on food as fighting left them trapped in their neighborhoods.

'We are hungry and cold. There's no food, and we're running out of fuel for heating,' said Tamir, an unemployed construction worker who hid with his family in their basement as heavy gunfire rocked the city. 

Syrian state television has regularly shown some areas of Homs, a city of one million, looking peaceful. But activist video on the internet shows other areas looking like a war zone of empty streets, crumpled bodies and blasted buildings. – Reuters