More than a quarter of a million people have been killed in Syria's brutal conflict since it began with anti-government protesters over four years ago, a monitoring group said today.

The Syrian Observatory for Human Rights said it has documented the deaths of 250,124 people, including at least 74,426 civilians.

The civilian toll includes 12,517 children and 8,062 women.

The overall death toll does not include some 30,000 people missing in Syria; among them 20,000 are said to be held in Syrian jails.

It also does not include thousands of loyalist forces held by rebel factions or by the Islamic State jihadist group.

The Syrian conflict began with anti-government protests in March 2011 before spiralling into a multi-front war across the country after a brutal crackdown by President Bashar al-Assad’s regime.

At least four million Syrians have been forced to flee the violence to neighbouring countries and millions more are displaced inside the war-torn country.

Meanwhile, Syrian troops backed by Hezbollah and Iranian fighters launched an offensive south of Aleppo today, expanding their counter-attack against rebels across western Syria with support from Russian air strikes.

The assault means the army is now pressing insurgents on several fronts near Syria's main cities in the west, control of which would secure President Assad's hold on power even if the east of the country is still held by Islamic State.

Aleppo, a commercial and industrial hub near the border with Turkey, was Syria's largest city before its four-year civil war.

Control of the city, still home to two million people, is divided between the government and rebels.

"This is the promised battle," a senior government military source said of the offensive backed by hundreds of Hezbollah and Iranian forces which he said had made some gains on the ground.