The prime minister told the House of Commons he has no doubts that millions of dollars raised by IS, formerly known as the Islamic State of Iraq and Levant (Isil), from ransom payments is used to promote terrorism affecting the UK.
And he said Britain will continue with its policy not to pay ransoms in cases of terrorist kidnap, adding the UK needed to ensure other countries are also “good to their word”.
A video showing the beheading of American journalist Steven Sotloff also included IS extremists threatening a British hostage with death.
Speaking during Prime Minister’s Questions, Mr Cameron said: “There’s no doubt in my mind that the many tens of millions of dollars that Isil has raised from ransom payments is going into promoting terrorism, including terrorism affecting our own country.
“At the G8, I launched an initiative to try to get other countries to sign up to a very clear doctrine that in the case of terrorist kidnap, no ransom should be paid.
“Britain continues with this policy, America continues with its policy but we need to redouble the efforts to make sure that other countries are good to their word.”
Militants from the Islamic State group carried out a mass killing of hundreds of Iraqi soldiers captured when the extremists overran a military base north of Baghdad in June, a leading international watchdog said.
The incident at Camp Speicher, an air base that previously served as a US military facility, was one of the worst atrocities perpetrated by the Islamic State group in its lightning offensive that seized large swaths of northern and western Iraq.
According to Human Rights Watch, new evidence indicates the Islamic State fighters killed between 560 and 770 men captured at Camp Speicher, near the city of Tikrit — a figure several times higher than what was initially reported.
“These are horrific and massive abuses, atrocities by the Islamic State, and on a scale that clearly rises to the crimes against humanity,” Fred Abrahams, special HRW adviser, told reporters in the northern city of Irbil.
The al-Qaeda-breakaway claimed in mid-June that it had “executed” about 1,700 soldiers and military personnel from Camp Speicher.
The group also posted graphic photos that appeared to show its gunmen massacring scores of Iraqi soldiers after loading the captives onto trucks and forcing them to lie face-down in a ditch, arms tied behind their backs.
The grisly images, meant to sap the morale of Iraqi security forces, and the number of slain troops could not be confirmed at the time. Human Rights Watch said in late June that analysis of photos and satellite images showed that between 160 and 190 men were killed in at least two locations between June 11 and 14.
After the incident, the soldiers were listed as missing, prompting their families to stage demonstrations in Baghdad in an effort to pressure authorities for word on their sons’ fate. On Tuesday, dozens of angry family members stormed into the parliament in Baghdad’s fortified Green Zone after scuffling with security guards, causing commotion and arguing with lawmakers. They also forced the speaker to call a session yesterday on the missing soldiers.
The Human Rights Watch statement said the revised figure for the slain soldiers was based on analysis of new satellite imagery, militant videos and a survivor’s account that confirmed the existence of three more “mass execution sites.” The number of victims may well be even higher as more evidence emerges, the New York-based watchdog said.
“Another piece of this gruesome puzzle has come into place, with many more executions now confirmed,” said Peter Bouckaert, at Human Rights Watch.