Islamist extremism is rising in the Philippines, prompting Christians to sound the alarm, the World Watch Monitor (WWM) reported.

As if to underscore the rising security threats in the Southeast Asian country, the Islamic State (ISIS)-linked Abu Sayyaf extremist group beheaded German hostage Jurgen Kantner on Monday in Sulu province. This was confirmed by the Philippine government after a video showing the execution was monitored by the intelligence group SITE, according to ABS-CBN News.

A local pastor, who requested anonymity, told WWM that around 3,000 Islamists recently held a meeting on one island.

"The villagers – Christian or not – are all in fear. At night, the Islamists coerce men into going to the mosques. Some men have never been seen again," the pastor said. "When they see you out on the street, they'll come get you. One man who went to the mosque on 6 February never came back home."

The pastor said he found out recently that the Islamists threatened to burn a church being built by the Seventh Day Adventist and force the workers and Christians in the area to leave.

Meanwhile, the Philippines and Germany both condemned on Monday the beheading of Kantner, a 70-year-old German, by the Abu Sayyaf after the group failed to get the $600,000 ransom they demanded, Reuters reported.

Kantner had appealed for the authorities to pay the ransom to save his life.

Jesus Dureza, an adviser to Philippine President Rodrigo Duterte, said officials had exhausted all efforts to save Kantner but to no avail.

In a statement, the Philippine military revealed that some soldiers trying to rescue Kantner and other kidnap victims were also killed in clashes with the militants.

"Our operations in the past several days and nights using all our resources were unrelenting. We lost some of our best men in the process, because we value the life of Mr. Kantner and that of the others who have fallen prey to this terrorist group," the military said.

In Berlin, Chancellor Angela Merkel also condemned the "hideous attack" as she called for a united international front against terrorism.

Last year, the Abu Sayyaf also beheaded two Canadians.

Kantner and a companion were reportedly taken captive by the militants in November while sailing on a yacht near Sabah, eastern Malaysia, and brought to Jolo. His female companion was shot dead when she tried to resist the militants, according to Reuters.