The battle for Aleppo between Syrian government troops and rebel forces that has raged for more than four years has come to an end.

Rebel fighters fled from their final strongholds on Monday following a three-week government advance into the war-torn city, according to the Syrian Observatory for Human Rights.

The battle for Aleppo began in mid 2012.

Hundreds of thousands of civilians had found themselves besieged in the eastern-held, rebel half of the city in recent months, as government forces attempted to push out the insurgents.

In recent weeks, Russian-backed Syrian forces had continued their intense advance to regain full control of Aleppo, one of Syria's largest cities.

As the government made ground, civilians continued to flee from rebel-held parts of the city to safer zones.

Approximately 20,000 people were estimated to have left rebel-held areas on Saturday alone.

In the past four years, more than 20,000 residents of Aleppo province have been killed, according to the Syrian Observatory.

More than 80% of these are estimated to have been in rebel-held areas.

Civilians in Aleppo have been subject to almost constant bombardment for over a year, with the battle frequently characterised by civilian casualties.

Almost all of Aleppo's hospitals were eventually bombed out of action, while basic foods and supplies remained dangerously low throughout recent moments.