“The abhorrent murder of David Haines once again shows how important it is that the international community mobilise against Daesh [the Arabic acronym of IS], an abject and cowardly organisation,” a statement by the presidential palace said.
“France declares its solidarity with David Haines’s family and with Great Britain,” it went on.
The IS released a video, entitled A Message to the Allies of America, late Saturday appearing to show a masked man beheading Haines, declaring that he had to “pay the price” for Britain arming Kurdish peshmerga militias in their fight against it.
The 44-year-old was working for French NGO Acted when he was kidnapped in Syria in March 2013.
Acted said it was "deeply appalled and horrified" by the killing in a statement Sunday.
The IS earlier killed two American journalists, James Foley and Steven Sotloff, and has now threatened to kill another British hostage, Alan Henning.
President François Hollande visited Baghdad and the Iraqi Kurdish capital, Arbil, on Friday and is hosting an international conference aimed at building a coalition to fight the IS in Paris on Monday.