More than 55,000 people were killed in Syria in 2015, including over 2,500 children, bringing the total death toll of the deadly civil war in its fifth year to 260,000. 

As many as 55,219 people, including 20,977 civilians, were killed in Syria in 2015, monitoring group Syrian Observatory for Human Rights said. 

Terror groups such as the Islamic State, al-Nusra Front and other affiliates saw 16,212 casualties, while 7,798 Syrian rebel fighters were killed. However, the monitoring agency said that the real death toll among these groups could be much higher as these groups did not disclose their losses. 

The Syrian regime forces also suffered heavy losses, with 8,819 soldiers and officers being killed this year. More than 7,000 pro-regime militia men were also killed in Syria. 

Syria's civil war began in 2011 with anti-government  protests during the wave of the Arab spring in the region, but the conflict escalated to deadly proportions over five years, leading to the rise of deadly terror groups such as the Islamic State, which controls swathes of territory in the country and is responsible for several executions and terror attacks. 

While 2015 saw fewer deaths than 2014, when 76,021 people had been killed, the monitoring agency called for international intervention to end the war. 

"We in the Syrian Observatory for Human Rights consider the silence of the international community for the war crimes and crimes against humanity committed in Syria encourages the criminals to kill more and more Syrian people," the Britain-based agency said in its report on Thursday. 

On 18 December, the United Nations Security Council  approved a resolution to bring about a political solution in Syria even as differences remained over the fate of Syrian President Bashar al-Assad. 

The Security Council called for a political process to establish "credible, inclusive and non-sectarian governance," within six months and for free and fair elections to be held within 18 months.