The year 2014 was the deadliest yet in Syria's four-year conflict, with more than 76,000 people killed, activists say.
The British-based Syrian Observatory for Human Rights said 17,790 of the dead were civilians, including 3501 children, the BBC reported.
Meanwhile more than 15,000 died in conflicts in Iraq in 2014, making it that country's worst year since 2007.
Much of the violence comes as a result of advances by Islamic State and other militants groups in the two countries.
US-led air strikes against IS militants, fighting between government troops and rebels in Syria, and sectarian violence in Iraq have also accounted for large numbers of deaths.
The air strikes continued on Thursday (local time), with 17 against IS targets near the Syrian cities of Raqqa, Kobane and Deir al-Zour and 12 near the Iraqi cities of Falluja, Mosul and Sinjar.
Meanwhile, Syrian President Bashar al-Assad marked the new year with a visit to front-line troops in the Damascus suburb of Jobar.
Mr Assad's Twitter and Facebook accounts showed pictures of him talking to two soldiers by a tank and shaking hands with a third.
The Syrian figures cannot be independently verified but the Observatory says its draws on sources all across the country.
It said a total of 76,021 had died, up on 2013's toll of 73,447 and bringing the total number of deaths since the conflict began in 2011 to more than 200,000.
Of the 2014 deaths, the Observatory said at least 22,627 were government soldiers or members of pro-government militias, almost 17,000 were militants from groups including IS and al-Nusra Front and more than 15,000 were from moderate rebel groups and Islamist factions. Civilians accounted for the rest.
On the same day, the Iraqi government published its figures on casualties in 2014.
It said 15,538 people had died and more than 22,000 were injured during the course of the year.
British-based non-government organisation (NGO) Iraq Body Count gave an even higher figure of 17,073 civilian deaths.
The NGO said there was a "new brutality on the ground and renewed attack from the air", with IS and the Iraqi army causing thousands of casualties, and US-led air strikes also responsible for civilian deaths.
The year began with the government losing control of Falluja and parts of Ramadi in western Iraq.
In June, a huge offensive by IS left large parts of northern Iraq in militant hands, including the second city Mosul.