Hundreds of Palestinian prisoners in Israeli jails have joined a fellow inmate on a hunger strike, after human rights groups reported the original protester's life was in danger.

Khader Adnan, widely believed to be a leader of the Palestinian group Islamic Jihad, has been refusing food since he was detained on December 17, without trial or charge.

Jamil Khatib, Adnan's lawyer, told Al Jazeera on Sunday that an appeal against his detention will likely be decided by an Israeli military court on Monday.

On Thursday, Adnan appealed his detention without charge before an Israeli military judge sitting in a special session in hospital.

His hunger strike, longer than any Palestinian prisoner before him, according to Palestinian officials, is in protest over what he calls his unjust detention and mistreatment by Israeli authorities.

Human Rights Watch on Saturday called on Israel to "immediately charge or release" him.

Sarah Leah Whitson, Middle East director at Human Rights Watch, said in a statement that Israel should "immediately end its unlawful administrative detention" of Khader Adnan and "charge or release him".

Administrative detention

Adnan was arrested from his house in the occupied West Bank on December 17, and given a four-month administrative detention order by an Israeli military court on January 10.

Hundreds of Palestinians, protesting outside Ofer Prison in the West Bank in solidarity with Adnan on Saturday, were dispersed by Israeli soldiers using rubber bullets and tear gas.

An Israeli military spokesman said that protesters had "hurled rocks at security forces".

In addition, two Israelis and two Palestinians were arrested in a separate rally for Adnan in the West Bank village of Beit Omar, the military and activists said.

There are currently some 310 Palestinians in administrative detention, a procedure that allows the Israeli military to hold prisoners indefinitely without charge or trial.

"Israel should end, today, before it's too late, its almost two-month-long refusal to inform Adnan of any criminal charge or evidence against him," Whitson said.

Mistreatment allegations

On Friday, Robert Serry, the UN Special Coordinator for the Middle East peace process, called on Israel "to do everything in its power to preserve the health of the prisoner and resolve this case while abiding by all legal obligations under international law".

Adnan’s wife, Randa, complained that medical staff were treating him badly after she visited him in hospital on February 7, the first time since his detention.

His health is deteriorating, she said, adding that a doctor had "mocked him when he asked for water and said that he should also stop drinking water".

"A lot of the hair on his face and head has fallen off. He has not been allowed to shower or wash during all his time in detention, nor is he allowed to wear warm clothes in this cold weather," Randa said.