Abu Sayyaf are the prime suspects after gunmen attacked a Vietnamese ship, killing one crew member and abducting seven.
Coastguard and marines rescued 17 Vietnamese crew members from the MV Giang Hai, which was attacked on Sunday (February 19) near Baguan Island in Tawi-Tawi, an area infested by the Islamic State affiliated Abu Sayyaf terror group.
Pham Van Hien, head of the safety department of Pham Hai shipping company, the Vietnamese owner of the cargo ship, which was carrying cement from Indonesia to the Philippines, said the captain was among those abducted.
The gunmen destroyed some of the ship’s equipment, but the 10 remaining crew members still managed to maintain control the ship, Mr Hien added.
He said the company had informed the IMB Piracy Reporting Centre in Malaysia to seek its help in securing the return of the kidnapped crew members.
Abu Sayyaf is believed to be equipped with fast boats and high-tech navigation devices, which has been making the seas around the Sulu peninsular some of the most dangerous waters in the world.
Last year a German couple were attacked on their yacht, with the woman shot, and the husband still held to ransom.
The group has also targeted domestic seafarers, such as in January this year when eight fishermen were slaughtered in cold blood off the coast of Zamboanga City. (See report here.)
Before Sunday’s attack, the militants were believed to be holding 27 hostages, including Mr Kantner, who they have vowed to behead on Saturday unless a ransom of 30 million pesos is paid.
Other captives are Indonesian, Malaysian, Vietnamese, Filipino, Dutch and Japanese.
The number of maritime kidnappings hit a ten-year high in 2016, with waters off the southern Philippines becoming increasingly dangerous, the International Maritime Bureau (IMB) announced last month.
While the overall number of pirate attacks has declined in recent years, the IMB said 62 people worldwide were kidnapped for ransom at sea last year compared to only 19 in 2015 and nine in 2014.
“The kidnapping of crew from ocean-going merchant vessels in the Sulu Sea and their transfer to the southern Philippines represents a notable escalation in attacks,” the IMB said in its report.
Defence Minister Delfin Lorenzana said the military had been ordered to end threats from Abu Sayyaf within six months, with more troops to be poured into the area.
As the clock ticks for Mr Kantner, a renewed assault involving special forces was announced last week.
Links between Abu Sayyaf and the so-called Islamic State have been confirmed recently, with intelligence suggesting that IS fighters are seeking a new base in the Philippines as the noose tightens around their strongholds in the Middle East.
It is also believed that funding is flowing from the Arabic world to Abu Sayyaf, hidden among the thousands of remittances that are sent from overseas workers in the region every day.