Here are some key statistics:
- Huge death toll -
The Syrian Observatory for Human Rights said last week it had recorded the deaths of 312,001 people since the conflict began with anti-government protests in March 2011.
Those killed included 90,000 civilians, of which 16,000 were children, the Britain-based monitor said.
In a country with a population of some 23 million before the war, the United Nations estimates around 6.6 million people have been internally displaced by the fighting.
And almost one million Syrians are besieged by the warring parties, according to UN aid chief Stephen O'Brien.
In November, he said there had been a steep increase in what he called this "deliberate tactic of cruelty", in particular by the Syrian regime.
- Refugees -
The war has forced 4.8 million people to flee Syria, according to the United Nations High Commissioner for Refugees.
Neighbouring Turkey has taken in more than 2.7 million Syrians, the UNHCR says, and is now the main host country.
It is followed by Lebanon, with more than one million Syrian refugees, according to the UN.
Jordan, which the UNHCR says has taken in 655,000 Syrians, says it has accepted considerably more, at 1.4 million.
At least another 228,000 Syrians have taken refuge in Iraq and 115,000 in Egypt, the refugee agency says.
Signup to our newsletter and follow us on Facebook and Twitter!
Syrian refugees have in increasing numbers travelled to or attempted to reach Europe, making the perilous journey overground or by sea.
- Imprisoned, tortured -
In August, Amnesty International said Syrian authorities were committing torture on a "massive scale" in government prisons.
It said more than 17,700 people are estimated to have died in custody since the conflict began.
But it said the real figure is much higher, pointing to tens of thousands of forced disappearances.
The Observatory, for its part, says at least 60,000 people have died over five years from torture or harsh conditions in regime prisons.
The monitor says half a million people have passed through regime jails since the start of the conflict.
And several thousand have died over the same period in prisons run by rebel groups or jihadists, it says.
In February 2016, UN investigators accused the regime of "extermination" in its jails and detention centres.
- Economy in ruins -
Experts say the conflict has set Syria's economy back by three decades, with almost all its revenues cut off and most of the infrastructure destroyed.
The education and health systems are in ruins.
In 2015, a coalition of non-governmental organisations said Syria was living almost without electricity, with 83 percent of lighting no longer working.
More than 80 percent of the population lives in poverty, according to a study published in April 2016 by the United Nations and Britain's Saint Andrews University.
The study also said the Syrian economy contracted by 55 percent between 2010 and 2015.