Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights told the AFP news agency that family members of government employees were among those killed near the city.

Fighters from the jihadist group are closing in on Palmyra, which is a UNESCO world heritage site.

Irina Bokova, the head of UNESCO, has called on Syrian troops and extremists to spare Palmyra, saying it "represents an irreplaceable treasure for the Syrian people, and the world".

The city, which is nicknamed "the pearl of the desert", is home to millennia-old tombs, colonnades and Roman temples.

Its art and architecture blends Greco-Roman techniques with local traditions and Persian influence.

IS would see such treasures as targets.

The group has already been filmed destroying artefacts in the Iraqi cities of Nimrud and Hatra.

Since the IS offensive began early on Wednesday, more than 138 combatants - 73 soldiers and 65 jihadists - have been kille

Before the latest killings, there were reports of at least 26 civilians being killed by IS, including 10 by beheading, for collaborating with the Syrian regime, the Observatory said.

Talal Barazi, the governor of central Homs province, where Palmyra is located, said the situation was "under control", with army reinforcements sent to the city and bombing raids being conducted on IS positions.

Meanwhile, IS has raised its black flag over the main government compound in the western Iraqi city of Ramadi - giving them nearly full control over Anbar's provincial capital.

Iraq's government has said the city has not fallen yet and that a major counter-offensive has begun.

The battle for Ramadi has been raging since last year, but IS renewed their push for the city in April, and launched a fierce offensive overnight that included six suicide car bombs to reach the city centre, Reuters reported.

Iraqi officials have also said IS has gained substantial control over the Beiji oil refinery.

But the US military command leading the fight against IS has insisted its strategy is working and the militants' recent takeover of the government compound and the refinery are fleeting gains feeding the IS propaganda machine.

Marine Brigadier General Thomas D. Weidley, chief of staff for Combined Joint Task Force Operation Inherent Resolve, said: "We believe across Iraq and Syria that Daesh is losing and remains on the defensive."

Daesh is the Arabc acronym for IS.