BEIRUT (AFP) - Islamic State in Iraq and Syria (ISIS) extremists have blown up the famous Arch of Triumph in the ancient Syrian city of Palmyra, said an activist and a monitoring group, as the militants press their campaign to tear down the treasured heritage site.

"The Arch of Triumph was pulverised. IS has destroyed it," said Mr Mohammad Hassan al-Homsi, an activist from Palmyra.

Citing sources on site, the Syrian Observatory for Human Rights monitoring group said the militants had destroyed the arch because of ornaments on its columns the group considered idolatrous.

Unesco's director-general on Monday condemned the destruction, saying "extremists are terrified by history and culture".

Ms Irina Bokova said the United Nations' scientific and cultural body would make every effort to ensure that the perpetrators of the destruction are brought to justice.

ISIS militants blew up the arch, dating from between 193 and 211 AD, as part of their sustained campaign of destruction of heritage sites in areas under their control in Syria and Iraq.

The Palmyra site is listed on Unesco's World Heritage List and before this year, attracted an annual 150,000 tourists.

"This new destruction shows how extremists are terrified by history and culture - because understanding the past undermines and delegitimises their claims - and embodies an expression of pure hatred and ignorance," Mr Bokova said.

She said Palmyra "symbolises everything that extremists abhor - cultural diversity, dialogue between cultures (and) the encounter of peoples of all origins".

"There will be no impunity for war criminals, and Unesco will make every effort, at its level, and in close cooperation with the International Criminal Court, to make sure that the perpetrators of the destruction are tried and punished," Mr Bokova said.

Known as the "Pearl of the Desert", the oasis town of Palmyra is situated about 210km north-east of Damascus and became famous as a stopping point for caravans travelling on the Silk Road.