Some residents sobbed as the Islamist militants broke down the 'sacred door' of one of the northern Malian city's three ancient mosques after they wrecked seven tombs of Muslim saints over the weekend.

Exclusive video footage obtained by AFP shows turbaned men chanting 'Allahu Akbar' (God is great) while smashing a mausoleum with pick-axes in a cloud of dust, the mud-brick tomb showing gaping holes in the side with rubble piling up alongside it.

The Organisation of Islamic Cooperation condemned the destruction, saying in a statement the sites were "part of the rich Islamic heritage of Mali and should not be allowed to be destroyed by ... bigoted extremist elements."

Ansar Dine (Defenders of Faith) believe the shrines to be idolatrous and have threatened to destroy any mosques housing the remains of the ancient saints, prompting an outcry from government and the international community.

"The Islamists have just destroyed the door to the entrance of the Sidi Yahya mosque... they tore the sacred door off which we never open," said a resident of the town on Monday morning.

A former tour guide in the once-popular tourist destination said: "They came with pick-axes, they cried 'Allah' and broke the door. It is very serious. Some of the people watching began crying."

Another man, a relative of a local imam (religious leader), said he had spoken to members of Ansar Dine, which occupied the city and the rest of northern Mali in the chaos following a coup in Bamako three months ago.

"Some said that the day this door is opened it will be the end of the world and they wanted to show that it is not the end of the world."

The door on the south end of the mosque has been closed for centuries due to local beliefs that to open it will bring misfortune.

It leads to a tomb of saints, however the Islamists appeared unaware of this as one witness said if they had known "they would have broken everything."

According to the website of the UN cultural agency (UNESCO) Sidi Yahya is one of Timbuktu's three great mosques and was built around 1400, dating back to the city's golden age as a desert crossroads and centre for learning.

The fabled city, which became a metaphor for a mythic, faraway place, is considered one of the centres from which Islam spread through Africa in the 15th and 16th centuries.

Algeria condemned the destruction of tombs which "constitute a homage and a recognition by the local people to the saints and scholars who contributed to the flourishing of Islam in the region and to the spread of the values of tolerance and spirituality."

The three mosques formed the 'university' of Timbuktu, also known as the "City of 333 Saints". Timbuktu is also home to 16 cemeteries and mausoleums.

Ansar Dine began their campaign of destruction after UNESCO put Timbuktu on its list of endangered world heritage sites.

"God is unique. All of this is haram (forbidden in Islam). We are all Muslims. UNESCO is what?" spokesman Sanda Ould Boumama said on Saturday.

He said the group was acting in the name of God and would "destroy every mausoleum in the city. All of them, without exception".

Pleas have poured in for the Islamists to halt the destruction, reminiscent of the Taliban blowing up the giant Buddhas of the Bamiyan valley in Afghanistan -- an ancient Buddhist shrine on the Silk Road -- in 2001 after branding them un-Islamic.

International Criminal Court prosecutor Fatou Bensouda on Sunday warned that the destruction could amount to a war crime.

"My message to those involved in these criminal acts is clear: stop the destruction of the religious buildings now," Bensouda told AFP in an interview in Dakar.

"This is a war crime which my office has authority to fully investigate."

United Nations chief Ban Ki-moon deplored the destruction of tombs, with his spokesman Martin Nesirky quoting him as saying: "Such attacks against cultural heritage sites are totally unjustified."

In a matter of months Mali has gone from one of west Africa's stable democracies to a nation gripped by deadly chaos.

A March 22 coup eased the way for Tuareg separatist rebels -- descendants of those who founded Timbuktu in the fifth century -- to carry out the armed takeover of an area larger than France they consider their homeland.

However the previously unknown Ansar Dine group fighting on their flanks seized the upper hand, openly allied with Al-Qaeda in the Islamic Maghreb, and have since pushed the Tuareg from all positions of power.

The international community fears the vast desert area will become a new haven for terrorist activity and the Islamists have threatened any country that joins a possible military intervention force in Mali.