Philippine Army soldiers keep watch at a military checkpoint in Sulu province on Feb. 27 after the militant group Abu Sayyaf posted a video claiming to show the beheading of a German hostage. (Ben Hajan/European Pressphoto Agency)

An Islamic State-affiliated group in the Philippines posted a video on Monday purporting to show the beheading of a 71-year-old German hostage after a deadline for his ransom had expired, a monitoring group reported.

The SITE Intelligence Group said the video was posted by the Abu Sayyaf group on its Telegram social media account and follows Sunday’s deadline for about $600,000 in ransom.

The authenticity of the video could not be confirmed independently, but a German government spokesman called the slaying a “sad certainty.” Abu Sayyaf has carried out similar beheadings of captives in the past, and the latest video brought swift condemnation from authorities in the Philippines.

“We grieve as we strongly condemn the barbaric beheading of yet another kidnap victim,” said Jesus Dureza, an adviser to Philippine President Rodrigo Duterte.

The video shows a confused-looking Jürgen Kantner surrounded by armed and masked men in camouflage. “They kill me now,” Kantner says before a man with a curved knife saws his head off. The severed head is displayed for the camera.

Kantner last appeared in a video on Feb. 14, when Abu Sayyaf militants demanded the ransom and threatened to kill him if the money was not delivered.

Kantner was seized by the extremists in November, sailing along the coast of the southern Philippines. His wife, Sabine Merz, was killed in the assault on their 53-foot yacht, Rockall.

Abu Sayyaf militants told local media that Merz was killed after she opened fire on them while trying to defend the boat.

In Germany, a statement by government spokesman Steffen Seibert said Kantner was “barbarically killed” by his captors, suggesting that German experts had determined the video to be authentic.

Seibert said Chancellor Angela Merkel “condemns the abhorrent deed” and called on nations to “stand together and fight” terrorism.

In 2008, Merz and Kantner were kidnapped by Somali pirates and held for nearly two months before a ransom was paid.

Abu Sayyaf has taken hostages for decades — and killed captives — as part of a rebellion it has claimed to be waging on behalf of the Muslim minority in the predominantly Roman Catholic Philippines. The government has deemed Abu Sayyaf a terrorist organization, one that operates more like a criminal gang, engaging in kidnapping for ransom, extortion and drug trafficking, among other crimes.

In April, the severed head of a 68-year-old Canadian held by Abu Sayyaf was left on a street in the southern Philippines, five hours after a ransom deadline.

In November 2015, Abu Sayyaf militants decapitated a Malaysian hostage on the same day that the Malaysian prime minister arrived in Manila for a summit.

Abu Sayyaf — a collection of militant splinter groups — has been weakened by expanded military and police operations over the past decade, but it retains footholds in jungle hideouts used as bases for sporadic attacks and kidnappings. The group had claimed an alliance with al-Qaeda, but it recently publicly proclaimed allegiance to the Islamic State.

The group, which is holding more than 20 foreign captives, has increasingly turned to kidnapping and ransom to raise funds.

A kidnapped German couple was released in 2014 for a reported ransom of $5.1 million.

Murphy reported from Washington. Stephanie Kirchner in Berlin contributed to this report.