A fleet of at least 30 fishing trawlers crewed by slaves is being hunted off the coast of Papua New Guinea as the true extent becomes apparent of the trafficking of Burmese men by a massive Thai-run criminal syndicate operating throughout the East Indies.

Immigration officials have so far intercepted one of the fishing vessels, called the Blissful Reefer, and rescued its trafficked crew. Another 33 Thai trawlers thought to be crewed by slaves are being tracked in fishing grounds off the south coast of Papua New Guinea, known locally as the Dog Leg.

The trawlers are thought to be linked to a huge trafficking operation that was disrupted on the isolated Indonesian island of Benjina in March, liberating hundreds of enslaved fishermen – although a large number of boats loaded with slaves managed to escape.

Analysis of the trafficking operation reveals that the fish, which were originally heading for Thailand’s huge export-oriented seafood trade, are entering global supply chains, with some almost certainly destined for Britain.

It has also emerged that another, much larger, fleet of fishing boats crewed by slaves has been identified on the Indonesian island of Ambon – 1,200 miles to the west and once an important destination in the region’s spice trade. Officials from the International Organisation for Migration (IOM) believe that a further 240 Thai fishing vessels are moored there, along with a total of around 1,000 slaves. To date, the crews of around 70 fishing vessels have been interviewed by IOM officials on the island, resulting in the rescue of some 350 Burmese slaves who will be repatriated to Burma (Myanmar). Accounts from a handful of former Burmese slaves who have already arrived home say hundreds of men remain unaccounted for.

Paul Dillon, a Jakarta-based IOM official, told the Observer: “We’ve interviewed the men from over a third of the 240 vessels in the port and discovered over 350 victims of trafficking, virtually all of whom are from Myanmar. If the pattern holds and we’re finally able to get access to the remaining men, we could be looking at up to 1,000.”

However, Dillon said local corruption had obstructed attempts to examine the remaining boats: “We are hoping they will see the light, understand that we are on a humanitarian, not a law-enforcement, mission, and let us get in there, assess and rescue these men and get them back home to their families.”

The findings and potential scale of slavery in Ambon has prompted the IOM to look at extending its investigation to ascertain how many other slave fishermen are being forced to work in Indonesia – an archipelago of more than 17,500 islands, of which just 922 are permanently inhabited.

“The Ambon experience has stirred us up to want to look at other parts of the country,” said Dillon. “Currently we don’t know where else in the country there are large numbers of fishing vessels standing by. Many of the islands are very remote.”

Fishermen preparing for departure at the port in Benjina. Photograph: Dita Alangkara/AP

Meanwhile, the hunt for the Thai fishing vessels in the narrow, dangerous straits of the Dog Leg will continue this week as the Blissful Reefer is impounded in the port of Daru in Papua New Guinea. The eight crew members of the vessel, rescued on Monday, have been found to be trafficking victims. George Gigauri, the IOM’s chief of mission in Papua New Guinea, said: “They are trying to locate an approximate area where the vessels are, and then narrow it down exactly. The search is becoming more targeted, although it is difficult.”

The boats are suspected of being part of a massive transnational Thai trafficking operation that until recently operated from the Benjina fisheries weigh station in eastern Indonesia.

In November, an investigation by Associated Press discovered hundreds of forced labourers, mainly from Burma, on Benjina. Some were filmed trapped in a cage, and many of those interviewed said they had been abused or had witnessed others being beaten – or in some cases killed.

Almost all described being kicked, beaten or whipped with toxic stingray tails if they complained or attempted to rest. Despite working 20- to 22-hour shifts and being forced to drink unclean water, they were either paid a pittance or went unpaid.

The discovery by AP led to at least 300 men managing to escape but, before help arrived at the island, boats loaded with slaves fled the region for new fishing grounds – some to the island of Ambon, others apparently to the Dog Leg.

The Burmese slaves are recruited to work in Thailand’s seafood business and are usually lured or tricked into leaving their country to go to Thailand, where they are then taken south and put on boats in Indonesia. Others, though, are kidnapped and forced to work.

Once in Indonesian waters – some of the world’s richest fishing grounds for species including tuna and prawns – the ships’ names and flags are changed to escape the authorities’ notice, although the captain of the trawler is usually a Thai national.

Thailand’s seafood industry is worth around £5bn a year, with the vast majority of its produce exported globally to satisfy the global appetite for cheap fish. The catches are deposited with a huge refrigerated “mothership”, which transports the fish back to Thailand. Dillon said: “Look, It’s a billion-dollar business. There are powerful interests out there who have been making a lot of money for many years off the backs of these men, through acts of great cruelty. It is not going to disappear overnight, but in Indonesia at this time there appears to be the will to break their business model.”

However, little is known of the size of the Thai criminal syndicates, of their connections or of how they manage to coerce and recruit so many slaves. Investigators are still searching for the nerve centre of the operation.

Gigauri said: “It’s still not clear to us how this operates. Where exactly are these boats registered? To which company do they report? Who does the recruiting? Where is the headquarters of this operation?”

Last year another Guardian investigation tracked the supply chain of prawns produced with slave labour to British and American supermarket chains. Another more recent inquiry linked Thailand’s fishing industry with the trafficking syndicates profiting from the misery of Rohingya migrants.