A hardline religious group occupying northern Mali has destroyed 15th-century mausoleums of Sufi Muslim saints in Timbuktu and have threatened to demolish the remaining 13 UNESCO world heritage sites in the fabled city, witnesses have said.

The attack by Ansar Dine group on Friday came just four days after UNESCO placed Timbuktu on its list of heritage sites in danger after the seizure of its northern two-thirds in April by rebels.

"They have already completely destroyed the mausoleum of Sidi Mahmoud (Ben Amar) and two others. They said they would continue all day and destroy all 16," Yeya Tandina, a local Malian journalist, said by telephone.

"They are armed and have surrounded the sites with pick-up trucks. The population is just looking on helplessly," he said, adding that the Islamists were currently taking pick-axes to the mausoleum of Sidi El Mokhtar, another cherished local saint.

"It looks as if it is a direct reaction to the UNESCO decision," Timbuktu deputy Sandy Haidara said by telephone, confirming the attacks.

The Islamist Ansar Dine group backs strict sharia, Islamic law, and considers the shrines of the local Sufi version of Islam idolatrous.

"Ansar Dine will today destroy every mausoleum in the city. All of them, without exception," Sanda Ould Boumama, the group’s spokesman, told AFP news agency through an interpreter from the city.

"God is unique. All of this is haram (or forbidden in Islam). We are all Muslims. UNESCO is what?" he said, declaring that Ansar Dine was acting "in the name of God".