Al-Qaeda is increasingly funding terror operations using at least US$125 million in ransom money paid largely by European governments to free hostages since 2008.
The payments totalled US$66 million in 2013 alone, according to an investigation by the New York Times on Tuesday.
While al-Qaeda's network was first funded by wealthy donors, "kidnapping for ransom has become today's most significant source of terrorist financing", said David Cohen, the Treasury Department's undersecretary for terrorism and financial intelligence, in a 2012 speech.
"Each transaction encourages another transaction," he said.
The organisation has openly acknowledged the windfall.
"Kidnapping hostages is an easy spoil, which I may describe as a profitable trade and a precious treasure," wrote Nasser al-Wuhayshi, the leader of al-Qaeda in the Arabian Peninsula.
Wuhayshi said ransom money - reaching about US$10 million per hostage in recent cases - accounts for up to half his operating budget.
The paper listed more than US$90 million paid to al-Qaeda in the Islamic Maghreb since 2008 - by Switzerland, Spain, Austria, and a state-controlled French company, and two payments from undetermined sources.
Somalia's al-Shabab insurgents received US$5.1 million from Spain, while al-Qaeda in the Arabian Peninsula received nearly US$30 million in two payments, one from Qatar and Oman, the other of undetermined origin.
Austria, France, Germany, Italy and Switzerland all deny paying ransoms for hostages. French nuclear company Areva also denied paying ransom.
However, last year a former senior French intelligence official said: "Governments and companies pay in almost every case.
"There is always a ransom or an exchange of some sort: money, the release of prisoners, arms deliveries."
The Times article cited former hostages, negotiators, diplomats and government officials in 10 countries and said the payments were sometimes hidden as development aid.
The US and Britain have notably refused to pay to free kidnapped nationals, the paper reported, with the result that just a few have been rescued in military raids or escaped.
However, the US has been willing to negotiate in some cases, including the recent trade of five Taliban prisoners held at Guantanamo in exchange for US soldier Bowe Bergdahl.