The rebel Free Syrian Army said the battle to "liberate" Damascus has begun, as fighting raged in the capital on Tuesday and the army countered it has the situation under control.

The influential Muslim Brotherhood, meanwhile, urged people in Damascus to rise up and back the rebels locked in a "decisive battle" against forces loyal to President Bashar al-Assad.

The appeal came after the Free Syrian Army announced it had launched a large-scale operation dubbed "the Damascus volcano and earthquakes of Syria."

Colonel Kassem Saadeddine, the Homs-based spokesman for the FSA's Joint Command, said "victory is nigh," and that the fighting would go on until the whole of the capital has been conquered.

"We have transferred the battle from Damascus province to the capital," said Saadeddine. "We have a clear plan to control the whole of Damascus. We only have light weapons, but it's enough."

"Expect surprises," he added, without elaborating.

The FSA, composed of army defectors and civilians who have joined the insurgency, relies on light weapons, in stark contrast to the regular army's heavy firepower.

Fighting has raged in Damascus since Sunday, with activists describing the escalation as a "turning point" in the 16-month uprising, with fierce clashes in several neighbourhoods.

Helicopters were deployed in Damascus for the first time, shooting heavily into neighbourhoods that have become centres of resistance.

For his part, a military officer in Damascus told AFP the army "had the situation under control and was chasing the terrorists seeking refuge in apartments and mosques."

The army had entered Al-Midan district and surrounded Zein al-Abideen mosque, while troops gave residents an ultimatum to leave ahead of an imminent attack, the officer said on condition of anonymity.

The official said that soldiers had entered the southern district of Tadamon at 4:00 am (0100 GMT) on Tuesday, where "there are still pockets of resistance."

The military source said "battles raged" in the eastern district of Qaboon, "where the majority of rebels were," adding that "33 terrorists were killed, 15 were wounded and 145 were arrested," in a reference to the rebels.

As the fighting raged, the Muslim Brotherhood said the people of Damascus must seize "this historic moment" by providing support for rebels in the Syrian capital.

"Prepare to become soldiers in the decisive battle. You will secure victory with your own two hands," the Islamist group said in a statement.

The battle must be "the gate to victory" in the uprising against Assad's autocratic regime.

"Our battle is now in Damascus... and this requires that we mobilise all the forces and all our efforts to secure victory," said the Brotherhood, a key component of the opposition Syrian National Council.

The Israeli army's intelligence chief said Syrian troops have moved from the Golan Heights towards conflict zones including Damascus.

"Assad has removed many of his forces that were in the Golan Heights to the areas of conflict," Major General Aviv Kochavi told MPs.

He also said that "radical Islam" was gaining ground and that Syria was undergoing a process of "Iraqisation," with militant and tribal factions controlling different areas.

According to the Syrian Observatory for Human Rights, gunfire was heard in Qaboon on Monday night.

On Tuesday afternoon, the Observatory reported the army had deployed reinforcements in Al-Midan as battles raged between rebels and troops.

It also reported 35 people were killed across Syria on Tuesday, including 16 civilians, 14 regime troops and five rebels. In Damascus, seven civilians and one rebel were killed.

The Observatory's toll reports cannot be independently verified. The watchdog has estimated more than 17,000 people have been killed in violence since the revolt broke out in March 2011.