Prime Minister David Cameron is summoning military and security chiefs for an emergency meeting in response to the beheading of a British hostage and a threat against another.

The Sunday meeting comes after Islamic extremists released a video showing the beheading of British aid worker David Haines and threatening another with death.

Haines is the third Westerner beheaded in recent weeks by the Islamic State group, which has seized vast swaths of territory in Syria and Iraq.

The 44-year-aid worker's family had issued a plea to his captors the day before the video was released urging the hostage-takers to contact them.

Haines was kidnapped in Syria last year.

His family has released a statement praising his charitable work and saying he was most alive when helping others.