Manila: The Philippines condemned on Monday the "barbaric beheading" of an elderly German captive by IS-linked Abu Sayyaf militants who posted a video of the killing after a deadline for a $600,000 ransom passed.
The video showed a machete-wielding militant behead Jurgen Kantner. The German had appealed for help twice in short video messages, saying he would be killed if ransom were not paid.
Jesus Dureza, presidential adviser on the Philippine peace process, said officials had exhausted all efforts to save Kantner, 70, who was held on the tiny southern island of Jolo.
"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."
He made no mention of the ransom demand.
German Foreign Minister Sigmar Gabriel said Berlin was in contact with the Philippine authorities, which had yet to confirm the authenticity of the video.
If the video turned out to be genuine, Gabriel added, it would be "one of the cruellest crimes one could imagine".
Abu Sayyaf decapitated two Canadians last year on Jolo but later freed their two companions, a Filipino woman and a Norwegian. The four were seized from a tourist resort. The ransom demand for the Canadians was 6.4 million each.
Abu Sayyaf's activities in recent years have been mainly banditry, but the Philippines believes IS has been in contact with members of the group's leadership with a view to gaining a foothold in Southeast Asia.
Abu Sayyaf, which means "bearer of the sword", has capitalised on decades of instability in the southern Philippines and generated tens of millions of dollars from piracy and ransom payments.
It uses the money to buy arms, high-powered boats and modern navigation equipment, allowing it to avoid detection and seize crew of slow-moving vessels with ease.
Kantner and his companion were taken captive in November while sailing on a 53-footer yacht near Sabah, eastern Malaysia, and brought to Jolo. His companion was shot dead when she tried to resist the militants.
Reports of Kantner's execution emerged on Sunday evening but the authorities cautioned that information they had received about his death was preliminary and drawn from sketchy, unverified intelligence. No body parts have been found.
Speaking on the sidelines of the U.N. Human Rights Council in Geneva, Philippines Foreign Secretary Perfecto Yasay said Manila was seeking technological help from allies to pinpoint the location of hostages.
"We will undertake our operations to make sure we give a premium to saving the lives of the hostages, and precisely because of this our task has not been easy," Yasay said.
"But we are prepared to crush them when the opportunity comes."
Thousands of troops have been deployed to do that in the Abu Sayyaf strongholds of Basilan, Tawi-Tawi and Jolo islands, yet despite air and ground offensives, piracy and kidnappings continue unabated. The rebels are entrenched in local communities, complicating operations by a military that is wary of incurring civilian casualties.
Abu Sayyaf is currently holding 26 hostages - 13 Vietnamese, seven Filipinos, a Dutch national, a Japanese, two Indonesians and two Malaysians, the military says.
Experts say the group is very unlikely to free any hostages without payment.