MANILA - A brutal group of Islamist extremists operating along the Philippines' porous southern borders with Malaysia has released a new video threatening to kill an elderly German hostage on Feb 26 if they are not paid 30 million pesos (S$854,000) in ransom.

In the video released on Tuesday (Feb 14) via the chat app Telegram, Mr Juerguen Kantner, 70, can be seen speaking in German, apparently pleading for his life.

A caption in the video said this was a "final ultimatum" from the militants holding Mr Kantner, who was abducted last year.

"If their demand of 30 million pesos is not met by Feb 26, at 3 pm, they will behead the hostage and post it on social media," it read.

Mr Kantner was abducted by Abu Sayyaf militants while he and his wife Sabine Merz were cruising a dangerous area of the Philippines last year. His wife was killed, purportedly when she tried to fight back with a shotgun.

The couple had been sailing the oceans for many years aboard their 53-footer, the Rockall. They were held for 52 days in Somalia in 2008 before their captors freed them, reportedly after a six-figure ransom had been paid.

The Abu Sayyaf, an insurgent group known for kidnapping foreigners for ransom, has defied more than a decade of US-backed military offensives against it and has conducted a lucrative kidnapping spree in recent years.

In 2014, the militants abducted another German couple off a yacht in the southern Philippines. They released the pair six months later after receiving what they said was the full ransom demand of US$5.1 million (S$7.1 million).