Germany's federal foreign office on Monday confirmed the murder after a video surfaced purportedly showing the captive's beheading at the hands of Islamist militant organization Abu Sayyaf.

"There is no longer any reasonable doubt that the missing German is alive. We are deeply shocked by the inhuman and gruesome action taken by the perpetrators," said a government spokesperson on Monday, according to German news agency dpa.

Authorities had received reports of his death on Sunday after the deadline for a ransom payment had passed. Earlier in the month, the Philippines military had launched airstrikes against Abu Sayyaf after they threatened to behead the man.

The group posted the video into online chat groups of supporters of the so-called "Islamic State." It was reposted by the militancy-monitoring group SITE. The footage appeared to show Jürgen Kantner slumped on a grassy lot with a man holding a knife to his neck.

"Now they'll kill me," the man said before he was murdered.

The Philippine military said they had not yet seen the video and that troops were trying to confirm the man's death.

Germany's Foreign Ministry spokesman, Martin Schäfer, said on Monday that "because of the short time frame" the foreign ministry couldn't verify the two-minute video and that the responsible authorities would review it. However, Berlin appeared very confident that the man was dead, with Chancellor Angela Merkel decrying the "adominable" act.

German Foreign Minister Sigmar Gabriel said his office was in contact with the Philippine authorities, saying if the video turned out to be genuine it would be "one of the cruelest crimes one could imagine."

Jesus Dureza, presidential adviser on the Philippine peace process, did not mention a ransom but said officials had exhausted all efforts to save Kantner.

"We grieve as we strongly condemn the barbaric beheading of yet another kidnap victim," Dureza said in a statement. "There must be a stop to this killing of the innocent and the helpless."

Military skeptical

The video "is not enough" proof that the execution took place, Brigadier General Restituto Padilla, a spokesman for the military, told German news agency dpa. He said he would not dignify the video by watching it.

The military had enlisted the help of local governments and the the Muslim separatist group Moro National Liberation Front to find the body, Colonel Edgard Arevalo, another spokesman for the military, told dpa. 

"Until we find the body of the victim, we still hold on to the hope (that he is still alive) and we will continue to conduct combat operations," he said.

Second time kidnapped

Kantner was captured in November when the group intercepted his yacht off the southern Philippines and shot his 59-year-old wife Sabine Merz after she fought back. Her body was left on the boat.

The couple had run into kidnapping problems before. While sailing in the same 53-foot yacht named Rockall in the Gulf of Aden in 2008, Somali pirates seized them and held them hostage for 52 days. They described their ordeal - which included a mock execution - in an interview following their release.

"My boat is my life and I don't want to lose her, nothing more. I don't care about pirates and governments," Kantner said in a 2009 interview with the AFP news agency while in Berbera, the main port in Somaliland.

Radical separatist group

Abu Sayyaf, a loose network of militants based in the Philippines' remote southern islands, had defied more than a decade of US-backed military operations seeking to eradicate it and has turned to lucrative kidnapping sprees in recent years.

On Sunday the Philippine military said it had launched airstrikes against the group after deadline for the delivery of a 566,000-euro (US$599,000) ransom passed with no updates.

Authorities believed Abu Sayyaf to be holding about 20 other hostages. In the past the group had freed several in return for ransom payments.

In 2000 the group abducted 21 tourists, including a German family, from a diving island in Malaysia. They were released after a ransom payment. In 2004 the group killed 116 people in an attack on a ferry in the Philippines. In 2014 the group abducted a German couple from their sailing yacht and released them after several months. This year, two Canadians were abducted and decapitated.

aw/kms (Reuters, dpa, AFP)