Amnesty International on Friday urged Israeli authorities to release Palestinian prisoner Hanaa al-Shalabi who has been on hunger strike for more than a month, saying she was "at risk of death."
The watchdog said Shalabi should be "immediately" released or charged "with a recognisable criminal offence" and "promptly" put on trial.
"The woman could die in detention after 37 days on hunger strike," Amnesty said in a statement.
Shalabi has been on hunger strike since her arrest in the northern West Bank on February 16, when she was originally ordered detained without trial for six months.
She was among more than 1,000 Palestinian prisoners released in October in a trade for Israeli soldier Gilad Shalit, held by Gaza Strip-based militants for more than five years.
According to Amnesty, the 30-year-old "is allegedly affiliated with the Islamic Jihad movement but has never been charged with a criminal offence."
Earlier this week she was moved to Meir Hospital in Israel.
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"Hanaa al-Shalabi was transferred this evening to Israel's Meir hospital after her state of health deteriorated," Palestinian prisoner affairs minister Issa Qaraqaa told AFP on Monday.
And Physicians for Human Rights-Israel said an independent doctor had examined Shalabi on behalf of the group "and concluded that her health had deteriorated considerably and her life was at risk."
A PHR doctor noted "a weakening of musculature, weight loss of 14 kilos (31 pounds), a very feeble pulse and a fall in blood sodium levels," it said.
Israeli Prisons spokeswoman Sivan Weizman told AFP on Monday Shalabi was returned to prison after being examined.
According to Amnesty's deputy director for the Middle East and North Africa, the Palestinian prisoner has told her lawyers and independent doctors who examined her of abuse by prison authorities.
Shalabi said "that Israel Prison Service officers have handled her violently while transferring her to hospital or the military court, and consistently pressured her to end her hunger strike," said Ann Harrison.
"If she remains in detention, she -- and all other Palestinian detainees who have joined her hunger strike in protest against the policy of administrative detention -- must be treated humanely at all times and receive regular access to medical treatment by an independent physician in a setting that respects the privacy of the doctor and patient," Harrison said.
Amnesty also cited unidentified reports saying "the Israeli authorities may be considering force-feeding her," a move the watchdog said "could constitute cruel and inhuman treatment."