JAKARTA (AFP) - Unidentified gunmen have kidnapped another Indonesian sailor in the perilous waters of the Sulu Sea where dozens of seafarers have been abducted by Islamist extremists in recent months, officials said Sunday (Aug 7).

The kidnapping took place Wednesday in waters off the north-east of Sabah state, on the Malaysian side of Borneo island, Indonesia's ambassador to Malaysia said.

"(The vessel) was intercepted by a boat carrying four armed men," ambassador Herman Prayitno told AFP.

The kidnappers took the Indonesian captain after failing to get the RM10,000 they demanded, Prayitno added. They released the two other crew, an Indonesian and a Malaysian.

Indonesia's foreign ministry said it was still trying to find out which group was responsible.

The kidnapping is the latest in a string of incidents in the Sulu sea where groups of armed men have ambushed fishing vessels and seized Malaysian and Indonesia citizens for ransom.

Ten other Indonesians kidnapped in recent months by the Philippine Abu Sayyaf extremist group are still being held.

Jakarta has banned Indonesian-flagged vessels from sailing to the Philippines and pushed for the implementation of joint maritime patrol in the vital waterway.

A handful of Malaysian sailors have also been kidnapped this year.

The Abu Sayyaf, who are based on remote and mountainous southern Philippine islands, this year beheaded two Canadian hostages after their multi-million-dollar ransom demands were not met, and in 2015 killed a Malaysian hostage.

The group is a loose network of a few hundred Islamist militants, formed in the 1990s with seed money from Osama bin Laden's Al-Qaeda network.