Syrian troops launched an offensive on several rebel-held parts Homs city on Saturday, an NGO said, pounding the districts with air raids and mortar fire.
"War planes carried out two consecutive raids against areas under siege in Homs city, as well as carrying out heavy shelling against the same districts," the Syrian Observatory for Human Rights said.
"The shelling is focused on the neighbourhoods of Khaldiyeh, Bab Hud, Hamidiyeh and Bustan al-Diwan in an unprecedented manner."
The group said regime forces were using mortar shells, rockets, tank shells and heavy artillery in the assault.
Observatory director Rami Abdel Rahman said troops were trying to storm Khaldiyeh and that fighting was underway around the Old City.
Khaldiyeh and the Old City have been under siege by the army 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.
A security source in Damascus confirmed fierce fighting was underway in Homs city.
"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."
"Terrorists" is the term the government in Damascus uses for rebels.
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.
Abdel Rahman 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."