Five female Nobel Peace Prize laureates have issued a statement in Northern Ireland castigating Osaka Mayor Toru Hashimoto over his recent justification of Japan's wartime system of sexual slavery.

The five, who gathered in Belfast for a three-day conference of the Nobel Women's Initiative, said Thursday they "condemn in the strongest possible terms the recent deplorable remarks" by Hashimoto.

"Sexual slavery in wartime is a form of gender violence and is today defined as (a) war crime," the laureates said in the statement.

In remarks widely disparaged at home and abroad, Hashimoto, who jointly leads opposition party Nippon Ishin no Kai (Japan Restoration Party), said anyone can understand that women forced to work at the Imperial Japanese military's wartime brothels were "necessary" for frontline soldiers during the war. The women are euphemistically referred to as "comfort women" in Japan.

The statement was issued by Jody Williams, a U.S. campaigner against land mines; Iranian human rights activist Shirin Ebadi; Mairead Maguire, who worked to end sectarian violence in Northern Ireland; Liberian peace activist Leymah Gbowee; and Rigoberta Menchu Tum, who promotes the rights of indigenous people in Guatemala.

The laureates, who were meeting to discuss the impact of war on women, received their peace prizes between 1976 and 2011.