BEIRUT (Reuters) - Syria’s air force carried out air strikes on Sunday on Douma city, an insurgent stronghold northeast of Damascus, with 18 people killed and at least 100 wounded including children and other civilians, the Britain-based Syrian Observatory for Human Rights said.

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Overnight, Syria’s military killed dozens of combatants in southern Syria linked to al Qaeda’s Nusra Front including three senior members, state media reported.

Four years into the conflict, the military has concentrated its forces in the south, the capital Damascus and areas along the country’s western coast.

Eastern areas are dominated by the ultra-radical Islamic State group which is also fighting Kurdish forces in the north.

The Observatory said more than 215,000 people have been killed since the start of the crisis in March 2011, around half of them civilians.

Syria’s army and allied combatants from Lebanon’s Hezbollah last month launched a large-scale offensive against insurgent groups, including Nusra Front and non-jihadist rebels.

“The army... targeted a gathering of terrorists from a group affiliated with Nusra Front in al-Sweiseh in the Quneitra countryside, killing dozens of terrorists including three of their leaders,” state news agency SANA said regarding the overnight clash.

A statement from the army made reference to operations in Quneitra without providing specifics regarding casualties.

The Observatory said Syrian military helicopters had dropped barrel bombs in central Quneitra. They targeted an area where insurgent Islamic brigades and Nusra Front had been battling Hezbollah and Syrian militias for several weeks, it said.

Before the southern offensive, Syrian troops had lost control of large areas of countryside near Jordan as well as parts along the frontier with Israel near the Golan Heights, according to regional military analysts and diplomats.

Southern Syria is one of the last areas where mainstream rebels opposing President Bashar al-Assad have a foothold. They have lost ground to hardline Islamist militants in the war.

The Syrian government describes all insurgents in the civil war as hardline enemies of the state funded by foreign countries.