Myanmar's navy has briefly detained and turned back journalists near a migrant boat being held off the country's southern coast, according to witnesses, as officials remain guarded over what will be done with the people on board.

Before being turned away, Reuters reporters saw hundreds of migrants — some extremely thin — crammed on the deck of the converted Thai fishing boat that had been intercepted in the Andaman Sea on Friday and held in the waters off Leik island.

Some sat on two of the four Myanmar navy vessels standing off the fishing boat, which had been discovered carrying 727 migrants.

A naval officer, who declined to be named because he was not authorised to speak to the media, said the boat had been found drifting at sea with no fuel or anchor, and was taking on water.

"Their water pump was broken. If we hadn't found them, they may have died," the officer said.

The government initially labelled the migrants "Bengalis" — a term used to refer to both stateless Rohingya Muslims and Bangladeshis — but officials later said they believed most of those on board were from Bangladesh.

Myanmar has come under harsh criticism for its treatment of Rohingya, more than 100,000 have fled persecution and poverty in Rakhine State since 2012.

More than 4,000 Rohingya and Bangladeshis have landed in rickety boats throughout South-East Asia in the last month following a crackdown on human trafficking in Thailand.

Myanmar's sensitivity over the migrant crisis was evident by the way its navy responded to the arrival of the journalists' boat.

At one stage, a sailor leaned over the rail of a navy boat to point his rifle at the approaching journalists.

Reporters from Reuters and other foreign media were questioned and made to delete pictures and videos before they were ordered to return.

Myanmar officials have given little information on what it intends to do with the migrants.

"They are still on the boat. We are providing them with the necessary humanitarian assistance," government spokesman Ye Htut said.

Officials have said the boat would possibly be taken to Rakhine State, in the country's west, or to neighbouring Bangladesh.

Reuters