The fighting has left more than 270,000 people dead, according to the Syrian Observatory for Human Rights, a Britain-based monitor which relies on a large network of doctors and activist sources across the country.
Among those killed are around 80,000 civilians, including 13,500 children.
Far more people are feared dead, however, with an unknown number killed in detention at the hands of the government, rebels or jihadists.
UN investigators in February accused the regime of "extermination" in its jails and detention centres.
Also Read
And a Syrian aid group in January denounced the incessant bombing of medical facilities in the country, where it said 177 hospitals had been destroyed and nearly 700 health workers killed since 2011.
In January, the United Nations said that 13.5 million people out of a pre-conflict population of 23 million had been forced from their homes.
The charity Save the Children said this month that at least 250,000 children are living under siege, with many forced to eat animal feed or leaves to survive.
About 4.7 million Syrians have fled to neighbouring countries.
"It is the biggest population of refugees for a single conflict in a generation," Antonio Guterres, then chief of the UN refugee agency (UNHCR), said in 2015.
Most of the refugees are in neighbouring countries, notably Turkey, which has become the biggest host country with more than 2.7 million on its soil, according to UNHCR.
It is followed by Lebanon with over one million. More than two thirds of these live in "extreme poverty," according to the UN.