The Abu Sayyaf militants released an Indonesian sailor on Thursday who was held for 50 days on the Philippine island of Jolo, the military said.

The Filipino military said in a statement that the Al-Qaeda-linked Abu Sayyaf turned over the sailor, identified as Herman Manggak, to Moro National Liberation Front (MNLF) group who later handed him over to the authorities.

However, the terms of his release were not disclosed. The Indonesian ambassador said to Malaysia that initially, the extremists demanded RM10, 000 (S$3,300) for him after freeing two other crew members.

"The release of the victim is the result of relentless focused military operations, combined with efforts of different sectors, particularly the local government unit of Sulu and other stakeholders," Army brigade commander Arnek dela Vega told The Straits Times.

Brigadier-General dela Vega said they had no information on whether the ransom was paid for Manggak's release, but it is very rare that Abu Sayyaf group releases captives without the payment.

Manggak was kidnapped at gunpoint in the Sulu Sea close to the Malaysian border with the Philippines. His release comes days after the gunmen released a Norwegian captive and three other Indonesian hostages.

An army spokesman said the 30-year-old captive asked for food and was in high spirits after he was taken to an army base on Thursday.

The Abu Sayyaf group, linked to the Islamic State in Iraq and Syria (ISIS), is known for kidnapping people and demanding millions of dollars in ransom for their return. In recent months, numerous Indonesian and Malaysian seafarers have been kidnapped by the bandit group.

The Filipino military said they are still holding five other Indonesian citizens in their custody.

Recently, the group beheaded two Canadians whom they had kidnapped from a beach resort after a ransom deadline passed.

After one year of captivity, the militants released Norwegian resort manager Kjartan Sekkingstad a few weeks ago and three other Indonesian hostages were also released shortly after.

Analysts say that the group is mainly focused on a lucrative kidnapping business rather than any religious ideology.

The rise of hijacking incidents at sea has prompted Indonesia, the Philippines and Malaysia governments to try to coordinate maritime patrols in order to control such incidents. In a trilateral maritime security meeting in Bali, the countries agreed to let each other enter one another's waters in times of emergency.