BEIRUT (Reuters) - At least 23 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 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 in the Baba Amr district. One fired its machinegun and another appeared to be launch mortar rounds.

Gruesome video showed mangled bodies lying in pools of blood on a narrow street. Power lines had collapsed and cars were burnt and blasted, as if by mortar rounds. "What's happening is a slaughter," said Fadi, a resident living near the flashpoint Baba Amr neighborhood. He said it was being hit with mortar shells and heavy machinegun fire.

An armed insurgency is increasingly eclipsing civilian protests in Syria. Now many fear a slide toward a sectarian war pitting 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 Assad belongs. Fighting in Homs has intensified since a double suicide bombing in Damascus on Friday that killed 44 people.

Fadi told Reuters via Skype that trenches the army dug around the neighborhood in recent weeks had trapped residents and rebel fighters. "They are benefiting from trenches. Neither the people nor the gunmen or army defectors are able to flee. The army has been descending on the area for the past two days." Other residents said the fighters have still been able to inflict casualties on the army. "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 British-based Syrian Observatory for Human Rights documented names of those reported killed in Monday's clashes. It also reported three people killed on the outskirts of Hama, north of Homs, as security forces fired on protests. Further, it said, explosions had occurred in Douma, a Damascus suburb, as the army clashed with rebel fighters. France said it was "deeply worried by the continued deterioration in Homs" and urged Syria to allow monitors entry.