MANILA (Reuters) - Ten Indonesian crew members on board a tugboat were kidnapped by Abu Sayyaf militants in the Philippines at the weekend, officials in Manila said late on Monday.

Two Philippine military officials said the militant group had demanded an undisclosed ransom amount from the boat’s owners. The officials declined to be identified because they are not authorized to speak to media.

The crew were operating a privately owned Taiwanese tugboat on its way from Jakarta to Manila when it was hijacked near the Malaysian border.

The crew managed to call their employer to inform them of the hijacking, but the exact location of the incident remains unknown, officials said.

Indonesian officials said they were investigating the incident, without elaborating.

Abu Sayyaf, known for kidnappings, beheadings, bombings and extortion, is one of the most hardline Islamist militant groups in the Muslim south of the largely Christian Philippines.