The tugboat was attacked while towing a coal barge in Philippine waters of the Sulu Sea, Indonesia confirmed on Friday. Out of the 13 sailors on board, six were allowed to return home and seven were detained by the militants.

"This is the third time (this year) Indonesians have been held hostage by armed groups in the Philippines and this is something we can't tolerate," said Foreign Minister Retno Marsudi (pictured above).

"The government will do everything possible to free the hostages," she added.

Kidnapping specialists

Sailing through the waters between Indonesia and the Philippines has grown increasingly risky in recent months as the Abu Sayyaf militant group boosted attacks in the area.

The Philippines-based faction specializes in kidnapping foreigners and demanding high ransoms. The militant group recently beheaded two Canadian nationals after Ottawa failed to meet its demands.

A spokesman for the Foreign Ministry said on Friday that he could not confirm if Abu Sayyaf was responsible for the tugboat attack, which was first reported earlier this week.

However, wives of two of the missing sailors previously told reporters that kidnappers were members of Abu Sayyaff. The guerillas were demanding $5 million (4.7 million euros) for the crew's release, the women told a local television on Wednesday. They claim their husbands called them and asked them to notify the government, their employers and the public.

Both the Indonesian military and the police initially denied the reports.

Last month, Abu Sayyaf released 14 Indonesian sailors after holding them for several weeks. Jakarta denied paying any ransom.

Hostage freed after months of captivity

On Friday, the Islamist group released a Filipino woman they captured in September last year. The woman was a partner of one of the two murdered Canadians.

"She was released early morning somewhere in the province of Sulu and brought to the governor's house. I have no idea if ransom was paid," police chief Wilfredo Cayat said.

The militia is currently holding Japanese, Dutch and Norwegian citizens.

Responding to the wave of kidnappings, the governments of Indonesia, the Philippines and Malaysia agreed on boosting security in the restive seas, with the plans including joint patrols on the busy trade routes.

The patrols, however, are yet to start. The Manila government has also seen little success in their efforts to eliminate the group.

dj/sms (AP, dpa, Reuters, AFP)