Syria's armed forces battered rebel-held parts of Homs city on Saturday with air raids, shelling and a ground offensive that prompted the opposition to issue a desperate plea for foreign military intervention.
"Warplanes carried out two consecutive raids against areas under siege in Homs city, as well as carrying out heavy shelling against the same districts," said the Syrian Observatory for Human Rights.
The Britain-based Observatory described as "unprecedented" the all-out shelling concentrated on the neighbourhoods of Khaldiyeh, Bab Hud, Hamidiyeh and Bustan al-Diwan.
Regime forces were using mortar shells, rockets, tank shells and heavy artillery in the assault, said the monitoring group, which relies on a network of sources on the ground for its reports.
Observatory director Rami Abdel Rahman said troops were trying to storm Khaldiyeh and that fighting was underway around the Old City, two districts that have been under army siege for around a year.
Homs, the third largest-city in the country, was one of the first to join the revolt against President Bashar al-Assad's regime more than two years ago.
The opposition National Coalition called on fighters of the rebel Free Syrian Army to "close ranks in the face of Assad's recent brutal attacks on Homs".
"Syria is in danger of disintegration as Assad's forces supported by Iran, Russia and Hezbollah militias launch a fierce offensive on the city of Homs," it said in a statement.
"The National Coalition calls on battalions of the Free Syrian Army to come to the aid of Homs with all means possible.
"We also emphasise the need for immediate effective and decisive actions by 'Friends of Syria' including through establishing a no-fly zone and carrying out military strikes against Assad's basic military bases."
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A security source in Damascus confirmed fierce fighting was underway in Homs.
"Military operations never stopped in Homs, but their pace increases according to priorities," he told AFP.
"What's important is to cleanse those neighbourhoods of Homs that are in the hands of armed terrorists, particularly Khaldiyeh, Hamidiyeh and the Old City," he added.
"The army is advancing on all fronts but at varying speeds," he said, insisting: "Advances may be slow but they are real."
Syrian state television said government troops were making "great progress" in Khaldiyeh where they "eliminated large numbers of members of armed terrorist groups" -- the term used by the government to describe the rebels.
An activist in Khaldiyeh told AFP on Saturday afternoon that the fighting had subsided somewhat since the morning.
"Today was the beginning of a new offensive," he confirmed, describing air raids and shelling on the town.
"The situation is even worse than before, we're suffering shortages of basic materials such as rice and sugar," he told AFP over the Internet.
On Thursday, Syrian regime troops took the town of Al-Qariatayn, in Homs province as part of a wider campaign to gain ground in the central region.
The Observatory chief said the army appeared to be trying to "isolate the south of (central) Hama province and the north of Homs province to cut the supply lines between the two provinces."