Bamako, Mali (CNN) -- Elderly men were keeping watch Saturday over Timbuktu's main library after Islamists burned a tomb listed as a UNESCO World Heritage site.

The attacks Friday were blamed on Ansar Dine, a militant group that seeks to impose strict Sharia law.

The ancient city in Mali was captured by at least two separatist Tuareg rebel groups -- one of which is Ansar Dine -- in an anti-government uprising in the northern part of the country that began in January.

The rebels burned the tomb of a Sufi saint where people come to pray, said Sankoum Sissoko, a tour guide familiar with the place. He said the library and other heritage sites remained under threat....