Canadian Prime Minister Justin Trudeau's stance against paying ransoms to terrorists has reignited debate. (Getty Images)

One of the primary channels through which terrorist organizations get hard cash is ransom payments for hostages. Some, such as Abu Sayyaf, a group in the Philippines that aligns itself with the Islamic State and recently beheaded a Canadian hostage, have few other sources of funding and operate almost as mercenary outfits. Abu Sayyaf has kidnapped dozens of Filipinos and foreigners and holds 19 right now. It has made tens of millions of dollars off hostages since its founding in the 1990s and has spent the money on everything from weapons to speedboats.

Ransom payments are the ultimate dilemma for a government. Pay them and you fund a terrorist organization, while encouraging them to kidnap more people, perhaps especially from your country. Don't, and the hostage will most probably end up dead.

Different governments have made different choices. Those who pay ransoms, including Italy, Germany and France, try their best to keep that practice hush-hush. On the other side are the United States, Britain, Israel and Canada, which are accustomed to defending their no-ransom policies if their citizens are killed. On Tuesday, a day after his executed countryman's head was found in a plastic bag, Canadian Prime Minister Justin Trudeau went a step further and announced that he and British Prime Minister David Cameron would be calling on other countries to end ransom payments once and for all.

"We agreed that it is something that we are going to make sure we do bring up with our friends and allies around the world," Trudeau told reporters. "We need to make sure that terrorists understand that they cannot continue to fund their crimes and their violence [by] taking innocents hostage."

Trudeau's comments reignited a fraught debate that has no easy answers. There is no doubt that citizens of countries whose governments regularly pay ransoms are more attractive to kidnappers. In a 2014 investigation, New York Times reporter Rukmini Callimachi found that al-Qaeda had been paid at least 36 times between 2008 and 2014 by European governments — and that of Canada. The group made at least $125 million in those transactions. Far fewer Americans and Brits were kidnapped by al-Qaeda, and none were apparently released by ransom.

But questions about the fidelity of Canada, in particular, to its no-ransom policy didn't end with the Times investigation. On Wednesday, Gar Pardy, a former Canadian diplomat, wrote an editorial in the Ottawa Citizen in which he scoffed at any country's claim to have never paid ransom money.

"In the 11 years before I retired from the Foreign Service, I managed the release of more than 100 kidnapped Canadians in all parts of the world. All returned safely to Canada," he wrote. "I cannot provide details, but it can be said with certainty: If a kidnapped person has been released, then a ransom of some sort has been paid. To believe that kidnappers can have a change of heart is a fairy tale. And derring-do raids by special forces are largely fiction from the imagination of Hollywood."

This image, from an undated militant video, shows Canadians John Ridsdel, right, and Robert Hall. Canadian Prime Minister Justin Trudeau has confirmed that the decapitated head recovered in the Philippines was that of Ridsdel, who was taken hostage by Abu Sayyaf militants in 2015. Hall remains in captivity. (Militant video via AP)

Memos between U.S. officials that were released in 2011 by WikiLeaks also revealed that a ransom was paid to release Bob Fowler and Louis Guay, two Canadian diplomats who were captives of al-Qaeda in the Islamic Maghreb. The documents did not say who paid the ransom, however, nor specify the amount.

If not ransom, though, then what options are there for the families of kidnapping victims? If the hostage was American, it used to be the case that the family could be threatened with prosecution if it tried to raise money for ransom payments. But in June, the U.S. government softened its stance, dropping that looming threat, and shifted its policy toward sharing classified information when necessary and even dealing directly with hostage-takers on behalf of families of private citizens.

Another option that is almost never successful is a raid and rescue. In the high-profile case of American journalist James Foley, who was held captive by the Islamic State militant group in Syria, the U.S. military publicized, long after the fact, that it had attempted a rescue but found the raided location to be empty. Robert Hall and others who remain captives of Abu Sayyaf face equally slim chances of such a rescue.

One of Canada's largest newspapers, the Globe and Mail, reported Tuesday that Canadian security intelligence agents and Royal Canadian Mounted Police officers are "believed to be on the ground in the Philippines along with Canadian military personnel" but that security experts say it would still be near-impossible to mount a successful operation, given that Abu Sayyaf typically travels among the region's many small islands. How does a rescue unit reach a small island without being detected?