Syrian troops and rebels pour into Aleppo as both sides batten down for the long haul after 40 police were killed on day four of a pivotal battle in the nearly 17-month conflict.

Sustained gunfire and explosions were heard early Wednesday in areas of the Syrian capital hostile to the regime of President Bashar al-Assad, activists reported.

A blast and heavy gunfire came from the Baghdad Street, a main arterial route through the city, while the sound of battle was also heard in the Christian area of Bab Tuma, said the Local Coordination Committees that organise protests.

The LCC also said that the capital's southern suburb of Tadamun was hit by mortar fire at dawn.

Damascus residents heard explosions and intermittent gunfire late on Wednesday in several districts, notably in the southwestern neighbourhood of Kfar Sousa.

Fighting in that district broke out again on Monday after a lull when rebels attacked a regime forces checkpoint using rocket-propelled grenades.

Since July 20, the battle between the rebels and government forces has focused on the country's commercial capital Aleppo in the north.

On Tuesday 154 people -- among them 35 civilians, 63 soldiers and 27 rebels -- were killed in violence nationwide, around half in Aleppo.

Both troops and rebels poured into Aleppo on Tuesday as both sides battened down for the long haul after 40 police were killed on day four of a pivotal battle in the nearly 17-month conflict.

According to the Britain-based Syrian Observatory for Human Rights, more than 20,000 people have been killed since the uprising broke out in mid-March last year.

There is no way to independently verify the figure, while the UN has stopped keeping count.