Islamic State militants have destroyed three ancient tower tombs in the Syrian city of Palmyra, according to a government official in charge of antiquities.

Tower tombs, built on high grounds, are a particular feature of the Roman-era city.

Maamoun Abdulkarim, head of the Antiquities and Museums Department in Damascus, said the militants had destroyed tower tombs including the Elahbel tower, which dates back to 103 AD.

He said the destruction took place before the militants destroyed the two-millennia-old temple of Bel on Sunday, confirmed by UN satellite images.

The smaller Temple of Baalshamin was also destroyed days before, and the group posted images of the damage.

The militants claim ancient relics and sites of worship promote idolatry.

Mr Abdulkarim said his information was based on witness accounts and satellite images provided by the Boston-based American Schools of Oriental Research.

The ASOR Syrian Heritage Initiative said the IS group has destroyed seven tower tombs since the end of June over two phases. The last round of destruction occurred between August 27 and September 2, including the Tower of Elahbel, the most prominent example of Palmyra's distinct funerary monuments. Earlier, the tomb of Iamliku and Atenaten were also destroyed.

"Collectively, the damage to these tombs is not confined to a single area within the Valley of the Tombs, but instead it is distributed throughout various locations, leaving some towers destroyed and others still standing," the report said, showing a sequence of satellite imagery with the sites before and after the damage. "The reasoning for this differentiation is unknown."

The UN cultural agency UNESCO called the destruction of Palmyra an "intolerable crime against civilisation".