May was the bloodiest month so far this year in Syria, with at least 6,657 people killed throughout the war-ravaged country, the Syrian Observatory for Human Rights said Monday, according to AFP.
The toll includes at least 1,285 civilians, more than half of whom were killed in regime air raids, the Observatory said.
The UN's Syria peace envoy Staffan de Mistura condemned government aerial bombardment of civilian areas over the weekend, calling it "totally unacceptable".
The Observatory said rebels and Kurdish fighters made up 793 of those killed in May, while jihadists from factions including the Islamic State group and Al-Qaeda's Syrian affiliate Al-Nusra Front numbered 2,109.
Another 2,242 regime forces were killed, most of them soldiers, the Observatory said.
The toll also includes 208 non-Syrian pro-regime fighters, among them forces from Lebanon's Shiite movement Hezbollah, and 20 unidentified people.
The death toll represents an increase of more than 2,000 people compared with April, when 4,458 were killed, according to AFP.
May saw several fierce offensives against government forces.
ISIS seized the historic city of Palmyra on May 21, after a bloody nine-day offensive.
Rebel groups led by Al-Nusra Front secured gains against the regime throughout the northwest province of Idlib.
Since it began in March 2011, Syria's conflict has left 220,000 people dead and has forced millions to flee.
Outgoing UN aid chief Valerie Amos last week made a final appeal to world powers to take action to end Syria's four-year war, saying it had brought more despair than ever thought possible.