LONDON (AFP) - Relatives of two Britons murdered by Islamic State in Iraq and Syria (ISIS) militants urged people of all faiths to come together to fight the hatred espoused by the group, ahead of a memorial service for one of the men on Saturday.

Family and friends of Scottish aid worker David Haines will gather in a church in his hometown of Perth to remember the 44-year-old, who was kidnapped in Syria last year and beheaded in a video released last month.

Ahead of the private service, Ms Haines' brother Michael and Mrs Barbara Henning, whose husband Alan was also beheaded by ISIS militants earlier this month, penned an open letter urging people to come together to defeat the threat. "We are writing this letter because we will not allow the actions of a few people to undermine the unity of people of all faiths in our society," they wrote, in the letter published on The Guardian website.

They added: "David and Alan's killers want to hurt all of us and stop us from believing in the very things which took them into conflict zones - charity and human kindness. We condemn those who seek to drive us apart and spread hatred by attempting to place blame on Muslims or on the Islamic faith for the actions of these terrorists."

Two United States nationals, journalists Steven Sotloff and James Foley, have also been killed by the ISIS group since it took over swathes of Iraq and Syria earlier this year.

Meanwhile, British photojournalist John Cantlie is being held hostage and has appeared in ISIS propaganda videos.