The militant offensive on world heritage site Palmyra began on May 13 and has since left more than 350 people dead.

Fighters from the Islamic State of Iraq and Syria seized around a third of the ancient Syrian city of Palmyra on Wednesday after heavy clashes with the military and allied combatants, the Syrian Observatory for Human Rights monitoring group said.

Earlier on Wednesday, the monitoring group said fighting was raging near security buildings in northern Palmyra, including the regime’s local state security branch, and close to the city’s notorious prison, in the east.

In northwestern Idlib province, which is almost fully in rebel hands, the Observatory said 22 civilians were killed Tuesday in a regime air raid on a border village.

The militant offensive on world heritage site Palmyra began on May 13 and has since left more than 350 people dead.

“Both sides are firing mortar rounds on each other, and regime war planes are shelling the northern parts,” the Observatory said.

Mohammad, an activist originally from Palmyra, told AFP that the city was suffering from water shortages and intermittent electricity.

“A large number of people from the city’s north have been displaced into other neighbourhoods. Some are sleeping in the streets,” he said.

On Monday, ISIS militants seized two gas fields north of Palmyra that the regime had been using to generate electricity for areas under its control.

Antiquities officials fear that ISIS wants to destroy Palmyra’s pre-Islamic cultural treasures, which include colonnaded streets and ancient citadels.

The city is also strategically located at the crossroads of key highways leading west to Damascus and Homs, and east to Iraq.

In Idlib, a regime air raid on the opposition-controlled village of Darkush left at least 22 civilians dead on Tuesday.

Darkush lies on the Syrian-Turkish border, just 50 kilometres (30 miles) northwest of Ariha, one of the last remaining regime bastions in Idlib province.

Also on Tuesday, a rebel coalition including Al-Qaeda’s Syria branch seized control of the Al-Mastumah camp — the largest military base in Idlib.

A huge blast and fierce explosions rocked the site, and rebels swiftly overran the base as regime forces fled south to Ariha.

According to the Observatory and Syria’s state television, a teacher was killed and at least 20 students wounded Wednesday when mortar rounds landed on their school in Damascus.

Syria’s conflict began in March 2011 with peaceful anti-government demonstrations, but has evolved into a complex war that has left more than 220,000 people dead.

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