Islamic State has blown up three ancient tower tombs in the city of Palmyra, Syria's antiquities chief said on Friday, continuing the destruction of a World Heritage site that UNESCO has condemned as a war crime.

The militants, who have already destroyed two Roman-era temples in the city, blew up the tombs from between the years 44 and 103 AD, Maamoun Abdulkarim told Reuters.

He cited sources in Palmyra who confirmed the destruction of the tombs including that of Elahbel, built in 103 AD, which was one of the best preserved and stood four storeys high and had an underground floor.

Islamic State, which has declared a caliphate in land it holds across Syria and Iraq, captured Palmyra from Syrian government control in May.

The group has carried out several mass killings in places it has taken over and destroyed monuments it considers sacrilegious, publishing photographs or videos of its actions.

It beheaded the 82-year-old guardian of Palmyra's ancient ruins last month.

U.N. cultural agency UNESCO has said Islamic State's actions were war crimes aimed at wiping out evidence of Syria's diverse cultural history.

Activists say the group is keeping tight control on communications inside the city, making tracking events difficult.

In the past two weeks the group blew part of the Temple of Bel and Baal Shamin temple as well as a row of columns, a U.N. analysis of satellite images confirmed.