Human Rights Watch on Wednesday called on Gaza's Hamas rulers to release two Israeli citizens with a history of mental illness believed to have been detained by the militants, saying their extended detention and isolation was "cruel and indefensible."

In a detailed report based on interviews with the men's families and with Israeli and Palestinian officials, the New York-based group said Avera Mangistu and Hisham al-Sayed likely wandered into Gaza on foot and had no connection to hostilities between Israel and Gaza. Hamas has indirectly acknowledged holding them but will not provide confirmation until Israel releases dozens of its jailed members.

Until they are released, HRW demanded Hamas treat the men humanely and allow them to communicate with family.

"Hamas's refusal to confirm its apparent prolonged detention of men with mental health conditions and no connection to the hostilities is cruel and indefensible," said Sarah Leah Whitson, HRW's Middle East director. "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, but it refuses to divulge more information without its demands being met.

Mangistu, 30, a Jewish Israeli of Ethiopian descent who lived in the southern coastal city of Ashkelon, entered Gaza by crossing a barbed wire fence near the beach in September 2014. Al-Sayed, 29, an Israeli-Arab of Bedouin descent from the Negev desert, was seen walking across the Gaza border in April 2015. HRW said both were found unfit to serve in the Israeli military and had been institutionalized several times because of mental illness.

HRW said a third Israeli citizen, 19-year-old Jumaa Abu Ghanima, appears to have crossed into Gaza in July 2016, but it was not able to confirm this. The group called on Hamas to unconditionally disclose what it knows about the status of all three men, and whether they are in fact in its custody. Failing to do so amounts to enforced disappearance under international law, it said, which leaves detainees exceptionally vulnerable to torture and other abuse.

Whitson said that Mangistu and al-Sayed both suffer from serious mental illness and come from among the most marginalized communities in Israeli society.

"There is nothing patriotic or heroic in forcibly disappearing them," Whitson said.

Israel's Foreign Ministry and Hamas both declined comment.