At least 600 people are reported to have died after a powerful cyclone smashed into Bangladesh's coast, levelling villages and uprooting trees.
Hundreds of thousands of people were evacuated or sought safe shelter before the storm hit the coast from the Bay of Bengal, but some were left behind.
The true extent of the devastation remains unclear as the storm has blocked access to the affected areas.
The storm weakened on Friday as it passed through the capital, Dhaka.
As it was downgraded to a tropical storm, attention turned to assessing the devastation and distributing aid.
The damage from Cyclone Sidr, which has now moved well inland north-east of Dhaka, was worst on Bangladesh's southern coastal strip.
The government's disaster agency estimated the confirmed number of dead at 606 on Friday.
Tens of thousands of homes are thought to have been damaged and the recent crop harvest has also probably been destroyed.
The World Food Programme is sending emergency food rations for 400,000 people. The government, the Red Crescent and other NGOs are also sending teams.
More than 40,000 policemen, soldiers, coastguards and health workers have been deployed.
But amid a virtual national blackout, the authorities have been struggling to get food, medicine, tents and blankets to the affected areas.
River ferries are not running, roads are blocked by uprooted trees and Dhaka's main airport was forced to suspend operations.
The Home Ministry in Dhaka said several districts could still not be contacted as telephones and communications were cut and reports of casualties were confused.
Many people are thought to have been killed as falling trees levelled fragile houses made of thatch, wood and tin.
At least 150 fishing boats in the Bay of Bengal have failed to return to shore.
The UN Office for the Coordination of Humanitarian Affairs (OCHA) said 1,000 fishermen were missing, reported Reuters new agency.
Red Crescent officials have said at least three villages were flattened by the storm.
Dhaka residents told the BBC news website that buildings and roofs were shaken by fierce winds during the night, and that by morning power and water supplies had been cut.
"All night the wind has been raging so hard that I thought my window will shatter," said K Ashequl Haque.
Search for survivors
"We have mounted a search by civilians, army and police, and the casualty figures will rise," an official in Barisal, one of the worst hit districts, told Reuters.
The cyclone had roared in from the Bay of Bengal just before dusk on Thursday, generating winds of up to 240km/h (150mph) and driving rain.
It later blew past India's eastern coast without causing much damage, police and weather officials said.
Many people are taking cover in state-provided cyclone shelters
The storm triggered 5m (16ft) tidal surges in many of the affected districts. Rivers flowing into the Bay of Bengal were said to be swollen and rising.
Southern Bangladesh is often hit by cyclones, but experts say the latest one is a category four storm, the most powerful so far in the season.
Bangladesh developed a network of cyclone shelters and a storm early-warning system, after a cyclone killed more than 500,000 people in 1970.
Casualties from cyclones have been significantly reduced as a result, officials say.