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.

Four army defectors were killed by security forces in a town near the Turkish border, an activist network said. Nine soldiers killed in fighting in Homs were buried, state media reported.

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 on the Internet by activists showed three tanks in the streets next to apartment blocks in the Baba Amr district. One fired its main gun and another appeared to launch mortar rounds.

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 shelled by tank or mortar rounds.

What's happening is a slaughter, said Fadi, a resident living near the flashpoint Baba Amr neighbourhood. 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 neighbourhood 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.

TWO-WAY FIGHT

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 were defended by the Free Syrian Army, made up of defectors from the regular armed forces, who say they have tried to protect civilians.

There are many casualties, activist Yazen Homsi told the Avaaz opposition group from Homs. It is very difficult to access them and provide treatment as a result of the heavy shelling throughout the neighbourhood.

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.

It said explosions went off in Douma, a Damascus suburb, as the army clashed with rebel fighters. In a town near the Turkish border, four army defectors were killed by security forces, the Observatory reported.

GOVERNMENT TRANSPORT FOR MONITORS

France said it was deeply worried by the continued deterioration in Homs and urged Syria to allow monitors entry.

The initial 50 of 150 Arab League monitors left Cairo on their way to Syria on Monday, and some were due to go to Homs on Tuesday, a source at the group's headquarters in Cairo told Reuters. Their job will be to assess whether Assad is withdrawing tanks and troops from Syria's third largest city as promised.

The first group of about 50 Arab League monitors will be divided into five 10-man teams going to five locations.

The head of the team, the Sudanese General Mustafa Dabi, told Reuters that Damascus had been cooperative thus far.

Our Syrian brothers are cooperating very well and without any restrictions so far, he said.

But he added that Syrian forces would be providing transportation for the observers - a move which may anger the anti-Assad opposition and spark accusations of censorship.

Arab delegates said they would maintain the upper hand.

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 mandate 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.

Despite the scenes of ravaged streets, Syrian state television has been regularly showing other areas of Homs, a city of one million, looking peaceful.

The United Nations says at least 5,000 Syrians have been killed since the revolt, inspired by other Arab uprisings this year that have toppled three dictators, broke out in March. An estimated one-third of deaths have occurred in and around Homs.

The Syrian authorities blame the violence on foreign-backed armed Islamists who they say have killed 2,000 members of the security forces. After six weeks of stalling, Damascus signed a protocol this month to admit the Arab League monitors.

A source inside the opposition Syrian National Council said a growing number of its members are pushing to openly endorse armed insurrection against the government.

But they faced resistance from those in diplomatic contact with Western powers and calling for U.N. support -- such as creation of safe havens to protect protesting civilians.

Assad, 46, succeeded his father in 2000 to carry on 41 years of family rule. He has responded to popular calls to step down with a mixture of force and promises of reform, announcing an end to a state of emergency and promising a parliamentary election in February.

Arab League mission chief Dabi reached Damascus on Saturday while the capital was still reeling from suicide bombings the day before that killed 44, marking an ominous escalation of the violence in the heart of the Syrian capital.

Leading opposition groups such as the Syrian National Council say they suspect Assad's government carried out the Damascus bombings itself to prove to the world that Syria is facing indiscriminate violence by militant Islamists and to intimidate the work of monitors.

The explosions in Damascus carried the signature of the Syrian intelligence forces, the SNC's Paris-based president Burhan Ghalioun told Asharq Alawsat newspaper. But these terrorist operations will not discourage people from continuing the revolution to topple this regime no matter the sacrifices.

(Additional reporting by Mariam Karouny in Beirut, Ayman Samir and Marwa Awad in Cairo and Ayat Basma in Beirut; Writing by Douglas Hamilton; Editing by Mark Heinrich and Matthew Jones)