More than 55,000 people were killed in Syria in 2015, the country's fifth year of war, including over 2,500 children, a monitor said on Thursday.

The total number of dead since the beginning of the conflict had reached more than 260,000, the Britain-based Syrian Observatory for Human Rights said, though the number of those killed in 2015 was lower than the 76,021 people who died in 2014.

In 2015 alone, the monitor documented the deaths of 55,219 people, including 13,249 civilians. Among them were 2,574 children, it said.

As in previous years the dead were mostly combatants, including 7,798 rebels and more than 16,000 jihadists from the Islamic State group, Al-Qaeda affiliate Al-Nusra Front and associated organisations.

The group also documented the deaths of 17,686 regime forces, among them over 8,800 army troops, more than 7,000 Syrian pro-regime militiamen, and 378 members of Lebanon's Hezbollah movement.

A total of 1,214 foreign fighters from other countries, including Iraq, Iran and Afghanistan, died fighting for the regime, the Observatory said.

The group said it had recorded the deaths of an additional 274 people whose identities could not be established.

Since the war began in March 2011, the Observatory has documented the deaths of 260,758 people, including more than 76,000 civilians.

Over 45,000 rebels and more than 95,000 regime forces have been killed.

Another 40,121 jihadists have also been killed in fighting and air strikes including by a US-led coalition and Russian warplanes.

Syria's conflict began with peaceful anti-government protests but descended into a brutal civil war after a regime crackdown on dissent.