A British Islamist preacher has been recorded justifying beheadings of non-Muslim captives by the brutal Islamic State (ISIS) organization - and even suggests that all able-bodied non-Muslim men are legitimate targets for murder.

In a video obtained by the Middle East Media Research Institute (MEMRI), Minazur Rahman relates to the recent beheading of British aid worker Alan Henning by ISIS - an act which generated opposition even from many conservative Muslims, who noted that Henning was in Syria to help Muslim victims of the Assad regime's bloody campaign.

Jumping to defend ISIS's actions, Rahman - who was jailed for four years in 2007 for soliciting murder - claimed that under Muslim law (sharia) "a combatant is... any adult, mature, capable kaffir (infidel) man." 

"If he has no covenant of security with the Muslims, he is called a combatant... it's not 'did he fight,' it's about 'is he capable to fight'," he said.

That definition excludes women, children, the elderly and disabled, he added. Later in his speech, however, Rahman noted that women and children of non-Muslims should be taken "as slaves."

Clarifying his position, he said that simply being a citizen of a country "at war with Islam" would make a non-Muslim fair game - even if he was a humanitarian worker.

Turning to those who claim "that the original rules of Islam is peace and no war," he continued: "that's a misunderstanding... The original rules between Muslims and kuffar (infidels) is fighting and war" unless they convert to Islam or establish "a covenant with us."

Rahman did say that if some Muslims felt Henning qualified as a non-Muslim with a "covenant of security" and therefore unjustifiably killed, they should "go to the sharia court in the Islamic State."

"If he was killed wrongly therefore his family deserves blood money - that is 100 camels," he said.

The video was taken on October 22, three weeks after Henning's murder but more than two weeks before the beheading of aid worker Peter Kassig - who had apparently converted to Islam and taken the name Abdulrahman while in captivity.

Rahman's "explanations" for ISIS's brutality aside, even Kassig's status as a Muslim did not save him from being executed.

It is unclear whether the comments in the video qualify as incitement, or whether police will be launching an investigation into Rahman - who operates on the extremist fringe of Britain's Muslim community.

Thousands of European Muslims have reportedly flocked to join the Islamic State terror group in Iraq and Syria, at least several hundred from the UK - although some estimates put that figure at 1,000, and officials have admitted the precise figure is not yet known. 

But there also exists a considerable amount of sympathy for Islamists who choose not to travel to join ISIS's ranks - with several demonstrations featuring ISIS flag-waving fanatics hitting headlines in Europe over the past few months.