At least 3,000 people are feared dead and thousands are missing in India's Andaman and Nicobar Islands, after huge waves swept across low-lying areas.
Damage to the islands' jetties will hamper relief operations
Police chief Shamsher Deol said the islands, which lie close to Indonesia, were much worse hit than first thought.
Many of the remote islands remain under water, he said, and officials have been unable to make contact with others.
It is feared some indigenous tribes may have been decimated by the sea surges following Sunday's earthquake.
Fresh tremors registering up to 6 on the Richter scale hit the Andaman and Nicobar Islands early on Monday.
Mr Deol, speaking in the regional capital, Port Blair, said all the villages and the coastal road on the island of Car Nicobar had been washed away in Monday's massive waves.
"There are five or six islands where we have not established any contact at all," he told India's NDTV news.
"At a very conservative estimate I would say that 3,000 people are dead and as many missing."
Congress Party president Sonia Gandhi and defence minister Pranab Mukherjee flew into the region on Monday to assess the damage.
A local official travelling with Ms Gandhi said as many as 30,000 people were believed missing, according to the AFP news agency.
Port Blair councillor Pratima Madhukrishnan, in Madras, said there had been no news from Great Nicobar, home to 45,000 people and close to the earthquake's epicentre.
The BBC's Geeta Pandey, who is in Port Blair, says relief operations will be slow to reach many areas because the only access is by sea.
An estimated 94% of the jetties in the region have been damaged, making it difficult for boats to dock when they arrive.
In the capital, hundreds of people driven out of their homes by flood water have taken shelter in a school, she adds.
Coastguard officials reported seeing panic-stricken people in parts of the islands, which are still experiencing aftershocks and intermittent rough seas, Reuters news agency said.
Only 38 of the 572 islands, which lie about 1,500km (915 miles) east of India's mainland in the Bay of Bengal, are inhabited.
Among the population are six hunter-gatherer groups who have lived isolated from the outside world for thousands of years, including the Shompen aborigines, who number just 100, the Nicobari and the Sentinelese.