British Foreign Secretary Philip Hammond has denounced the slaughter of 21 kidnapped Egyptian Christians by Daesh.
The men, who were taken hostage in Libya, are shown in a video wearing orange overalls. They are forced to the ground and then beheaded.
Hammond said in a statement: "I strongly condemn the murder of 21 Egyptian Coptic Christians by ISIL-affiliated extremists in Libya. My thoughts are with the families of those killed.
"Such barbaric acts strengthen our determination to work with our partners to counter the expanding terrorist threat to Libya and the region.
"Acts of terrorism should not be allowed to undermine Libya's political transition.
"We remain fully supportive of the UN's efforts to build a national unity government for Libya and to bring a political solution to the ongoing security crisis.
"Those who support terrorists can have no part in this process."
The kidnapped Egyptians were Coptic Christians, seized in December and January from the eastern coastal town of Sirte - now under the control of Islamist groups.
Libya has been in turmoil since Muammar Gaddafi was overthrown in 2011, with two rival governments and a variety of militia groups battling for turf.