BAMAKO (Reuters) - Heavily armed Islamists bulldozed the tombs of three local Sufi saints near Mali's desert city of Timbuktu on Thursday, residents said, the latest in a series of attacks in the rebel-held north that critics say threaten its cultural heritage.

"They arrived aboard six or seven vehicles, heavily armed," said Garba Maiga, a resident of Timbuktu, listed by UNESCO as a World Heritage Site for its ancient shrines. "They flattened everything with a bulldozer and pulled up the skeletal remains."

Residents said the tombs destroyed included those of local saints Cheick Nouh, Cheick Ousmane el Kabir, and Cheick Mohamed Foulani Macina, several kilometers (miles) outside of the city gates. They said the rebels were from Ansar Dine, one of a mixture of Islamist groups now in control of northern Mali.

The destruction of the tombs follows an attack on another shrine outside Timbuktu at the end of September and several others in July.

A spokesman for Ansar Dine was not available to comment.

The al Qaeda-linked fighters say they are defending the purity of their faith against idol worship, though historians say their campaign of destruction is pulverising a valuable part of the history of Islam in Africa.

Sufi Islam, which reveres saints and sages with shrines, is popular across much of northern Mali.

Mali descended into chaos in March when soldiers toppled the president, leaving a power vacuum that led to Islamist fighters, some allied al Qaeda, seizing the northern two-thirds of the country.

The U.N. Security Council last week passed a resolution urging African regional groups and the United Nations to present a specific plan within 45 days for military intervention in Mali to help government troops reclaim the north.

But diplomats say the challenges of putting together an African force make it unlikely an operation could be mounted before March at the earliest.

(Reporting by Tiemoko Diallo; Writing by Richard Valdmanis; Editing by Jon Hemming; Editing by Jon Hemming)