Families of two British aid workers killed by the Islamic State group in Syria say people of all faiths should unite against the militants' "hateful acts."

In a letter to The Guardian newspaper, Barbara Henning and Michael Haines urged everyone "to find a single act of unity — one simple gesture, one act, one moment — that draws people together."

Barbara Henning's husband Alan and Michael Haines' brother David are two of the Western hostages beheaded on IS videos.

A memorial service is being held for Haines Saturday in his hometown of Perth, Scotland.

In a video message, Michael Haines remembered him as "a man full of kindness, open and caring, willing to cross the road to help others."

The militants have also killed two Americans and threatened a third.