Human Rights Watch is calling Hamas' detention of two Israeli citizens with a history of mental illness "cruel and indefensible."

In a report released Wednesday, the New York-based group says Avera Mangistu and Hisham al-Sayed likely wandered into Gaza on foot and had no connection to hostilities with Gaza's Hamas rulers. Hamas has indirectly acknowledged holding them but will not provide confirmation until Israel releases dozens of its jailed members.

HRW called on Hamas to release them, and to treat them humanely and allow them to communicate with family.

Sarah Leah Whitson, HRW's Middle East director, says "no grievance or objective can justify holding people incommunicado and bartering over their fates."

Hamas is also believed to hold the remains of two Israeli soldiers killed in the 2014 Gaza war.