Jonathan Powell, a veteran of the Northern Ireland peace process, warned in an interview that violent chaos in Libya will spread to its neighbours and to Europe and Britain if left unchecked.

Powell was speaking before news emerged on Sunday of the beheadings of 21 Egyptian Christians by Islamic State (Isis) fighters near Sirte and Monday’s retaliatory bombing raids by the Egyptian air force on Isis training locations and weapons stockpiles in Libya.

The brutal killings and the continuing flight of migrants via the Mediterranean coast are bleak reminders of the repercussions of the country’s breakdown. François Hollande, the French president, and Abdel Fatah al-Sisi, his Egyptian counterpart, called on Monday for the UN security council to meet over Libya and to take new measures.

“Libya is in a downward spiral that we need to reverse and turn into an upward one,” Powell told the Guardian. “There is a good deal of immediacy about this. But these things don’t happen overnight. You’ve got to rebuild trust that has been badly broken.”