Two of the crew were rescued by two merchant ships after Tuesday's ordeal off southeastern Sri Lanka, said Roshan Kumara from the southern fishing village of Kudawella. They were handed over to the navy and later admitted to hospital to be treated for cuts and other injuries.

Three crew members were still missing late Tuesday.

Kumara said the leader of his fishing boat collaborated with the attackers who arrived in another boat and discussed with them plans to going to Australia. Later they beat up the fishermen, and threw them into the sea where they managed to float for hours until their rescue.

The fishing boat with the attackers aboard has not been found.

Police spokesman Prishantha Jayakody said four other people have been arrested on shore accused of helping plan the hijacking. There were about 10 would-be illegal migrants on the boat, he said.

Sri Lanka's navy has arrested hundreds of people travelling illegally to Australia over the past months. The journey often takes migrants to Indonesia before they set said for Australia.

Australia is struggling to stop a surge of would-be refugees from countries including Afghanistan, Iran and Sri Lanka. Hundreds have died while attempting the dangerous sea voyage from Indonesia to Australia over the past year.