Floods caused by days of torrential rain, described by weather officials as unusually heavy and devastating, inundated at least a dozen out of Bangladesh's 64 administrative districts.

All major rivers including those flowing from the Himalayas through India have passed danger levels, flooding many villages and eroding vast tracts of land, leaving thousands homeless.

In the northern district of Bogra, hundreds of mud-walled houses collapsed.

Although the monsoon officially began only on June 7, already at least 30 people have been killed across the country in flooding which has damaged crops awaiting harvest and washed away dozens of fish farms, local officials said.

Farmers in other regions welcomed the early rains that had helped them till and sow their land with rice and jute. But they feared that more floods, which experts predict could hit again around mid-July, would damage the country's prime agriculture sector which accounts for more than 20 percent of GDP.

"Global warming, silting of the rivers and unplanned road construction have changed the routine of the flooding in Bangladesh," said Sajedul Karim, a senior director at the Flood Forecasting and Warning Center in Dhaka.

"Now people often suffer more from months of waterlogging because the floods cannot recede quickly," he told Reuters.

Weathermen said they felt the weather was behaving strangely, especially during the monsoon which lasts until mid-September.

"The impact of unusual weather often hits our agricultural output," Mirza Mufazzal Islam, senior scientific officer at the Bangladesh Institute of Nuclear Agriculture (BINA), told Reuters.

"Too warm or cold weather destroys land fertility, reducing crop yields," he said.