Four men were shot dead and more than 400 died from dehydration and drowning after the Thai army intercepted migrant workers from Bangladesh and Burma and left them to drift at sea, according to survivors' testimonies seen by the Guardian.

Indian coastguard ships have rescued about 400 dehydrated people and taken them to the Andaman and Nicobar islands. This week a survivor and three bodies were found on an uninhabited island. A boatload of 193 people was rescued by the Indonesian navy near Aceh.

According to survivors, whose accounts were recorded in coastguard and security reports in Port Blair, in the Andaman islands, they were brutalised, towed out to sea and left to drift, with little food and water, in boats with no engines.

Officials in Port Blair estimate that the Thai army could have had 1,000 "boat people" in its sights. The travellers had started out at Teknaf, a town south of Cox's Bazar and close to the Bangladesh-Burma border. Reports say only 600 survived. "All the survivors tell the same story. They say they were kept on an island, they were beaten up, some were shot dead, and then they were pushed out to sea," said Andaman's police chief, Ranjit Narayan. "But we have no way of verifying their story."

Thailand has denied the accusations, but has agreed to launch an inquiry.

Police will not let journalists meet the survivors held in Port Blair. But the Guardian has seen coastguard and security agency joint interrogation reports on the migrants, who set out in groups starting in November. According to these documents, most are Bangladeshi Muslims, but there are Muslims of the Rohingya ethnic group, from Burma's western Arakan state, who have been persecuted by the junta. All were heading for Malaysia to find work. Thailand was a transit destination.

One account was provided by Abdul Malik, 22, an unemployed scrap steel cutter from Bangladesh. Late last year a "job agent" promised him work in Malaysia. Malik, who can speak English, paid the agent 15,000 takas (about £150), and was put on a boat with 43 others.

After the group was caught, in the presence of Thai soldiers, the members were approached by "a Muslim middleman" who offered to take them to Malaysia if they paid 2,500 takas each. But no one had money and they ended up on a "forested island inside a barbed wire enclosure".

According to the South China Morning Post, the island was Koh Sai Daeng, in the Andaman sea, off Thailand's western coast. A tourist's photographs showed migrants alongside holidaymakers. The migrants were pictured lying in rows on the beach, and a wooden fishing boat was near the shore among the pleasure craft.

"We first thought they were seals, not human beings," said a tourist. "Some of them were trying to sit up and looked like they were complaining but were answered with a whip on the back or head. They were whipped at least eight or nine times."

The interrogation report says that "Thai army personnel used to torture them physically". It states: "Their hands were tied and they were beaten mercilessly several times and they were not even properly provided food and water."

On about 17 or 18 December "some senior Thai officials" went to the island. The report says: "In front of them, around 9pm on the same day, the uniformed personnel started shifting all the Bangladeshis/Myanmarese [Burmese] to one big wooden barge which had neither engines nor sails/oars. During this time four persons were shot dead randomly and their bodies thrown into the sea, and one juvenile aged around 14-15 yrs whose hands were tied was also thrown into the sea."

According to Indian officials, survivors said the teenager had protested loudly after being put in the wooden boat.

The vessel was moved off at about 9pm, with no engine, no oars and just two sacks of boiled rice and two gallons of boiled water. It was carrying more than 400 people. "It was towed for 18 hours in [a] north-east direction," says the report. The boat was then abandoned.

Days later, on 24 December, about 300 dehydrated men jumped into the sea to swim after seeing a light on the horizon. Only 11 made it to shore, on Little Andaman Island, after a two-day swim. Malik was one of them. The others drowned.

A coastguard ship rescued the 88 still on the boat. "Any delay and the hull would have sunk," said Satya Prakash Sharma, the coastguard inspector general for the Andaman and Nicobar islands. "It was taking in water and was badly waterlogged."

Kailash Negi, a coastguard commandant, said: "These are all poor people who were looking for work. But they were treated very harshly, inhumanely, and they were in a horrible condition."