The United Nations’ High Commissioner for Human Rights has condemned Israel for taking “punitive measures” against more than 1,000 Palestinian hunger-strikers calling for improved conditions in Israeli jails.

As the hunger strike entered its 38th day, Zeid Ra’ad Al Hussein slammed Israel for restricting access to lawyers and denying family visits, as dozens of protesters were transferred from prison to hospital as their condition worsened.

Almost 600 prisoners have been moved within the prisons to an observation ward, after the concerted effort – led by de facto Palestinian leader Marwan Barghouti – began refusing food on 17 April.

The hunger-strikers, who may still be force-fed, are calling for an increase in the number and duration of family visits, and improved access to healthcare.

They are also calling for an end to administrative detention, the controversial Israeli policy of holding suspects without trial or charge. Up to 500 Palestinian prisoners are currently held in administrative detention, according to the UN.

Zeid said: “I am especially alarmed by reports of punitive measures by the Israeli authorities against the hunger strikers, including restricted access to lawyers and the denial of family visits. The right of detainees to access a lawyer is a fundamental protection in international human rights law that should never be curtailed.”

Earlier this month the International Committee of the Red Cross said the right to family visits is enshrined in the Geneva Convention and “should never be restricted for punitive reasons”.