Sri Lanka's navy on Sunday rescued more than 100 Bangladeshi and Myanmar nationals from a sinking fishing vessel off the island's eastern coast, a spokesman said.

One passenger was found dead while many of the 138 plucked from the boat were dehydrated, he added, but it was not immediately clear where the vessel was headed or whether they were asylum seekers.

"We sent three ships for the rescue at a location 50 miles (80 kilometres) off the eastern coast of Akkaraipattu," navy spokesman Kosala Warnakulasuriya told AFP.

"Some have been admitted to a local hospital. Others are being taken to Colombo."

The early-morning rescue came amid stepped up naval patrols to check Sri Lankan fishing boats taking would-be illegal immigrants to Australia.

Sri Lankan authorities arrested more than 1,200 people trying to leave the island illegally last year. Many of those who make the perilous journey pay up to $3,000 for a place on trawlers run by people-smugglers.

Warnakulasuriya said the passengers rescued on Sunday identified themselves as Bangladeshi and Myanmar nationals.

"We have not been able to get much information about their port of origin or the destination, but we have asked the embassies of the two countries to help us with translations," he said.