Fifty monitors and 10 other officials from the Arab League arrived from Egypt on a private plane, the first international intervention on the ground to end nine months of violence between President Bashar al-Assad's troops and opponents.
Some monitors are due on Tuesday to visit Homs, scene of the worst violence, where there has been no sign of Assad carrying out a plan agreed with the Arab League to halt his offensive.
Amateur video posted on the Internet by activists showed 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.
Mangled bodies lay in pools of blood on a narrow street, the video showed. 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 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.
Four army defectors were killed by security forces in a town near the Turkish border on Monday, an activist network said. Nine soldiers killed in fighting in Homs were buried, state media reported.
Homs resident Fadi told Reuters via Skype that residents and fighters were trapped by trenches the army had dug around the neighborhood in recent weeks.
"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 gave his name 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 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 neighborhood."
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, it Observatory reported.
The Syrian government has banned most access by independent media, making it difficult to verify accounts of events.
GOVERNMENT TRANSPORT FOR MONITORS
Shortly before the monitors arrived on Monday, France said it was "deeply worried by the continued deterioration in Homs" and urged Syria to allow the monitors in.
The head of the observer mission, Sudanese General Mustafa al-Dabi, arrived in Damascus on Saturday. The League says the mission will eventually include 150 monitors.
"Our Syrian brothers are cooperating very well and without any restrictions so far," Dabi told Reuters.
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.
The first group of monitors will be divided into five 10-man teams going to five locations. Those that are expected to visit Homs on Tuesday will try to assess whether Assad is withdrawing tanks and troops from Syria's third largest city as promised.
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.
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 broke out in March, inspired by other Arab uprisings this year. 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.