GENEVA (Reuters) - The U.N. human rights chief on Monday decried a "worrying rise" in detentions of migrants in Greece and Italy and urged authorities to find alternatives to confining children while asylum requests are processed.

More than one million migrants, many fleeing Syria's war, have arrived in Europe through Greece since last year. More than 150,000 have come in 2016 so far - 38 percent of them children, according to United Nations refugee agency data. Italy has also set up mandatory detention centers.

"Even unaccompanied children are frequently placed in prison cells or centers 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 to the U.N. Human Rights Council in Geneva.

"Alternatives to the detention of children must be developed," he told the start of a three-week council session.

Zeid said it was possible for Europe to create "well-functioning migration governance systems", with fair assessment of individual requests for international protection while removing what he called "hysteria and panic from the equation".

Zeid deplored anti-migrant rhetoric "spanning the length and breadth of the European continent ... This fosters a climate of divisiveness, xenophobia and even – as in Bulgaria – vigilante violence".

In April, Bulgarian police arrested a local man who had posted video on social media showing how he tied up three migrants near the Turkish border.

The migrant influx has drawn a rising public backlash, in part because of strains it has imposed on housing and social services. Concerns have also been raised in some EU countries about their ability to integrate many mostly Muslim migrants.

(Reporting by Stephanie Nebehay; Editing by Tom Miles/Mark Heinrich)