Islamist rebels in northern Mali took hoes and chisels to the tombs of ancient Muslim saints in the city of Timbuktu for a second day, ignoring international pleas to halt their campaign of destruction.

After smashing three ancient tombs on Saturday, the Islamist militants who consider the World Heritage shrines idolatrous, set about wrecking four mausoleums at the cemetery of Djingareyber, a local journalist said.

Mali's government and the international community have expressed horror and outrage at the destruction of cultural treasures in the fabled city, an ancient desert crossroads and centre of learning known as the ‘City of 333 Saints’.

On Saturday the Islamists destroyed the tombs of Sidi Mahmoud, Sidi Moctar and Alpha Moya, and on Sunday attacked Cheikh el-Kebir's mausoleum as residents stood by helplessly.

Crying ‘Allahu Akbar’ (God is Great), the men circled the cemetery clasping tools such as chisels and hoes, but did not have construction vehicles that were used in Saturday's rampage.

"There are many of us watching them destroy the mausoleum. It hurts but we can't do anything. These madmen are armed, we can't do anything but they will be cursed that is for sure," the journalist said on condition of anonymity.

The cemetery is situated in the south of Timbuktu in the suburb of the eponymous Djingareyber mosque built in 1327. In addition to three historic mosques, Timbuktu is home to 16 cemeteries and mausoleums, according to the UNESCO website.

The Islamist fighters from Ansar Dine (Defenders of Faith) are among the Al-Qaeda-linked armed groups, which occupied the north of Mali in the chaos that emerged after a March coup in Bamako.

Their presence in Timbuktu prompted UNESCO to on Thursday list the city as an endangered site because of the continuing violence in northern Mali and in the wake of an attack on a 15th century tomb in May.

"God is unique. All of this is haram (forbidden in Islam). We are all Muslims. UNESCO is what?" a spokesman for Ansar Dine, 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’.

Mali's Culture and Tourism Minister Diallo Fadima Toure on Sunday urged the UN to take action to preserve her country's heritage.

"Mali exhorts the UN to take concrete steps to stop these crimes against the cultural heritage of my people," Toure told UNESCO's annual meeting in the Russian city of Saint Petersburg.

The attacks were reminiscent of the Taliban blowing up the giant Buddhas of the Bamiyan valley in Afghanistan – an ancient Buddhist shrine on the Silk Road and a world heritage site – in 2001 after branding them un-Islamic.

The Malian government has denounced the ‘destructive fury, comparable to war crimes’ as pleas poured in for a halt to the Islamist rampage.

UNESCO session chairwoman Yeleonor Mitrofanova told a meeting in Saint Petersburg that the destruction was tragic.

"I appeal to all those engaged in the conflict in Timbuktu to exercise their responsibility – for the sake of future generations, spare the legacy of their past," she said.

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

The 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.