The category four cyclone, the most powerful in the South Asian country in the past decade, hit the coast of Bangladesh late on Thursday, with winds reaching 240 km (150 miles) per hour, ripping up trees and leveling houses, as well as causing damage to crops, transport routes, communication and power supply lines. Many coastal districts were flooded.
More than 20 million people in Bangladesh have been left without shelter.
Mehamed Abdul Rob, president of the Bangladeshi Red Crescent Society, the Islamic equivalent of the Red Cross, said official statistics did not take into account the data on victims on coastal islands and in isolated areas.
Bangladesh authorities said on Monday that Sidr had killed 2,407 people. Meanwhile, local media put the death toll at higher than 3,000.
Rob said his organization's information is based on numerous reports coming from thousands of volunteers working directly in the storm-afflicted areas rather than on the estimates of rescue services, which sometimes have no possibility to reach disaster-stricken regions.
International efforts are under way to help victims of the storm and supply basic necessities to the affected areas.
Cyclones kill hundreds of people in Bangladesh annually. However, following devastating cyclones in 1970 and 1991, which claimed half a million and 140,000 lives respectively, the south Asian country introduced a more effective early warning system and built a better network of shelters.