Islamist militants with alleged ties to the Islamic State (ISIS) beheaded one of their foreign captives—Canadian John Ridsdel—on Monday, the Philippine and Canadian governments confirmed.

The Abu Sayyaf group had demanded a ransom payment of 900 million pesos, or about $19.2 million ($6.4 million each) for its three foreign hostages—a Norwegian and two Canadians, including Ridsdel, setting a deadline of 3 p.m. Monday, the Wall Street Journal reports.

The militants apparently executed Ridsdel after the deadline lapsed for receiving the ransom payment.

Canadian Prime Minister Justin Trudeau strongly denounced Ridsdel's "cold-blooded" murder. "I am outraged by the news that a Canadian citizen, John Ridsdel, held hostage in the Philippines since September 21, 2015, has been killed at the hands of his captors," Trudeau said in a statement released Monday.

"Canada condemns without reservation the brutality of the hostage-takers, and this unnecessary death. This was an act of coldblooded murder and responsibility rests squarely with the terrorist group who took him hostage," he said.

Ridsdel worked in the Philippines as a mining consultant.

Local police said they found the Canadian's severed head left by two men on a motorcycle along a street in Jolo town, Sulu province in the southern Philippines.

Earlier on Monday, Philippine President Benigno Aquino III ordered the military and police to intensify their operations against the militants and "effect the rescue of the hostages," the Philippine Daily Inquirer reports.

Earlier this month, the Abu Sayyaf lowered its original ransom demand of one billion pesos ($21.7 million) for each of the foreign hostages to 300 million pesos. They also rescheduled the deadline from April 8 to Monday, April 25.

In a two-minute video which circulated several days ago, the hostages—Ridsdel, fellow Canadian Robert Hall and his Filipina girlfriend Marites Flor, and Norwegian Kjartan Sekkingstad—said they were being held in Sulu. They said they had been told by their kidnappers that if the ransom demand was not met, one of them will be executed at 3 p.m. on April 25.

In a joint statement, the Philippine military and the police vowed to use "the full force of the law" to bring the Abu Sayyaf militants to justice.

Earlier this month, 18 soldiers were killed and 52 others were injured in an ill-fated attempt to kill or capture Abu Sayyaf leader Isnilon Hapilon on the island of Basilan.

On Friday, seven more government soldiers were injured in another firefight with the small but well-armed band of Muslim "bandits," as Philippine government authorities call them.

Last week, the Philippine military urged foreign governments not to pay ransom to "discourage this kind of growing industry."

Authorities estimate that Abu Sayyaf now holds more than 20 foreign captives from countries including Canada, Indonesia, Japan, Malaysia, the Netherlands and Norway.

In 2014 the group released two German hostages, apparently after ransom was paid.

Abu Sayyaf's leaders have sworn allegiance to ISIS and shown off the group's black flags in a number of videos posted online.

However, security experts have expressed doubts on Abu Sayyaf's ideological connection to ISIS, regarding them instead as a criminal organisation primarily engaged in kidnapping for ransom, rather than jihad.