The United Nations human rights boss on Monday decried a "worrying rise" in detention of migrants in so-called "hotspots" in Greece and Italy and urged authorities to find alternatives to confining children while their asylum requests are processed.

"Even unaccompanied children are frequently placed in prison cells or centres ringed with barbed-wire. Detention is never in the best interests of the child - which must take primacy over immigration objectives," Zeid Ra'ad Al Hussein said in a speech. "Alternatives to the detention of children must be developed."