May was the bloodiest month so far this year in Syria, with at least 6657 people killed throughout the war-ravaged country, a monitor has found.

According to the Syrian Observatory for Human Rights on Monday, the toll includes at least 1285 civilians, more than half of whom were killed in regime air raids.

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 2109.

Another 2242 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 2000 people compared with April, when 4458 were killed.

Several fierce offensives against government forces played out in May.

IS 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.