The assault came as President Bashar al-Assad and Grand Mufti Ahmad Hassoun, the highest Sunni religious authority in the country, were shown on Syria TV attending prayers at a Damascus mosque to mark the Prophet Mohammed's birthday.

Six rebels were killed by regime shelling and overnight firefights in the Jobar district of Homs, the Syrian Observatory for Human Rights said, adding that 31 soldiers, 16 rebels and 26 civilians had died in violence there since Sunday.

The Syrian Revolution General Authority, a network of opposition activists on the ground, said regime troops used heavy artillery and clashed with the rebel Free Syrian Army (FSA) in an attempt to storm the west side of the city.

A focal point at the outbreak of the revolt in March 2011, Homs remains a key point of contention in the war between Assad's regime and the rebels, who now hold large swathes of territory in the east and north.

For the past six months, loyalist troops have laid siege to several districts of Homs, including the Old City, leaving hundreds of families in dire humanitarian conditions, activists say.

"The Syrian regime has escalated its attack on Homs city and its environs in order to disperse the people on sectarian lines and achieve what it believes will be a final victory over Homs," said the Syrian National Council (SNC), a major opposition bloc in exile.

"The regime uses the most heinous criminal methods against human beings... shelling with heavy weaponry, blocking off areas to prevent the bare necessities -- food, medicine -- from entering, sending in sectarian militia to wreak havoc and kill, and finishing with massacres of entire neighbourhoods and villages."

The SNC called for a nationwide rescue campaign, for the "FSA all over Syria to aid their comrades in Homs with equipment and men" and for aid agencies to give priority to the trapped and displaced people of Homs.

Elsewhere on Thursday, air raids targeted the town of Daraya near Damascus, the rebel-held town of Yabrud far to the northeast and Aqraba near the airport road to the southeast, as fighting raged around opposition bastions across the eastern outer belt of the capital, the Observatory said.

Warplanes raided Albu Kamal on the Iraqi border in the east, areas of Idlib in the northwest, Daraa in the south and Hama in the centre as rebels clashed with troops near the Nayrab and Kweyris military airports in the northern province of Aleppo.

Fighting continued unabated in the northeast city of Ras al-Ain on the Turkish border, where 58 people have died over the past week amid clashes between hardline rebels and Kurdish fighters.