Hardline Islamists occupying northern Mali went on the rampage in Timbuktu on Saturday, destroying ancient tombs of Muslim saints and threatening to wipe out every religious shrine in the fabled city.

The onslaught by armed militants from the fundamentalist Ansar Dine group was launched just two days after UNESCO named the city an endangered world heritage site because of the unrest in the vast desert north of Mali.

"They have raped Timbuktu today. It is a crime," said a source close to a local imam in the town known as the "City of 333 Saints".

Witnesses said the Islamists had destroyed the ancient tomb of one revered Muslim figure after encircling a cemetery in the north of the Timbuktu, and were on the attack against another in the east.

"This is tragic news for us all," Alissandra Cummins, chair of UNESCO's executive committee, said in a statement to AFP in Russia, where the body is meeting this week, describing the attacks as "wanton damage".

"I appeal to all those engaged in the conflict in Timbuktu to exercise their responsibility."

Ansar Dine, one of the hardline Islamist groups which seized control of the vast desert north of Mali in the chaotic aftermath of a March coup in Bamako, said no site would be safe in Timbuktu.

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

The Ansar Dine spokesman suggested Saturday's action was in retaliation for the UNESCO decision to put the World Heritage site, a cradle of Islamic learning founded in the fifth century, on its endangered list on Thursday

"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 -- which wants to impose sharia law in the region -- was acting "in the name of God."

Witnesses in Timbuktu said the gangs had destroyed the mausoleum of a saint whose 15th century tomb was already desecrated in May by members of Al-Qaeda in the Islamic Magreb, another of the groups in control in the north.

"As I am speaking to you, Islamists from Ansar Dine have destroyed the mausoleum of saint Sidi Mahmoud," one witness told AFP.

"They are now in the process of destroying the mausoleum (of Sidi Moctar)," added a local journalist. "They have said they will destroy everything."

UNESCO said its decision to place both the town and the nearby Tomb of Askia in Gao on its List of World Heritage in Danger "aims to raise cooperation and support for the sites threatened by the armed conflict in the region."

Mali has been gripped by chaos since disgruntled troops swarmed the capital Bamako in the south in March and ousted the elected president of what had been seen as one of Africa's model democracies.

Islamic and tribal Tuareg groups seized on the power vacuum push government forces out of northern Mali, an area the size of France and Belgium, including Timbuktu and the cities of Gao and Kidal.

Mali's neighbours in west Africa have held several crisis meetings on the situation.

But on Friday, another Islamist militant group in the lawless north, the Movement for Oneness and Jihad in West Africa (MUJAO), threatened any countries which join a military intervention force to end the crisis.

UNESCO, the world's main watchdog over the safety of some of history's greatest treasures and most threatened cultural exhibits, first designated Timbuktu a heritage site in 1988.

As well as three historic mosques, Timbuktu has 16 cemeteries and mausolea, according to the UNESCO website.

It is also home to nearly 100,000 ancient manuscripts, some dating back to the 12th century, preserved in family homes and private libraries under the care of religious scholars.

At its height in the 1500s, the city, a Niger River port at the edge of the Sahara 1,000 kilometres (600 miles) north of Bamako, was the key intersection for salt traders travelling from the north and gold traders from the south.