Photos sent to the Refugee Action Coalition show the inflatable boats surrounding the vessel

A boatload of Sri Lankan asylum seekers who have been moored off the Indonesian island of Java since October say that the military is threatening to force them off the vessel, an advocacy group has said.

Nearly 250 Tamils holed up on the boat near the port town of Merak are refusing to disembark until Australia grants them asylum.

"The Indonesian navy is either preparing to remove people from the boat or is intimidating very vulnerable people," it said.

Citing information relayed from the boat, Ian Rintoul, a spokesperson for the council, said a number of Indonesian inflatable boats carrying navy personnel with weapons and in camouflage uniform had approached the vessel.

He said the Tamils quickly untied the rope when sailors aboard one of the boats tried to board the vessel.

Rintoul said the navy boats pulled back when they saw the asylum seekers taking photographs and with video cameras, but later returned to surround the vessel.

"We are extremely worried," the coalition quoted one of the Tamils as saying.

"The large boat is big enough to hold a large number of people if they physically remove us… This is the second time the navy boats have threatened us."

The Tamils were intercepted by the Indonesian navy en route to Australia in October.

They have said they do not want to go ashore in Indonesia because they fear they will be sent to an Indonesian detention centre and forced to wait years for resettlement.

Meanwhile one of the eight Tamils who left the boat some three weeks ago has disappeared after returning to Sri Lanka, the coalition said.

Gunasekaram Sujendran, 25, had earlier been held in Jakarta before returning to Sri Lanka after getting news that his mother was seriously ill.