The United Nations has called Israel to improve conditions for Palestinians in custody, expressing alarm over reports of “punitive measures by Israeli authorities” against hundreds of prisoners who are on hunger strike for nearly 40 days.

More than 1,500 prisoners launched the action on April 17 to press for an end to administrative detention and solitary confinement, as well as longer and more regular family visits, landlines installed in prisons and better healthcare.

The protest has continued without resolution and the health of hundreds of participating prisoners has “deteriorated “significantly”, Zeid Ra’ad Al Hussein, the UN high commissioner for human rights, said in a statement on Wednesday.

“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,” Zeid said.

READ MORE: How my father survived the hunger strike in Israel

There are currently 6,500 Palestinian political prisoners held by Israel, including more than 500 administrative detainees, according to Jerusalem-based prisoner rights group Addameer.

Administrative detainees are arrested on “secret evidence”, unaware of the accusations against them, and are not allowed to defend themselves in court. Their detention periods can be indefinitely renewed.

Zeid said the right of detainees to access a lawyer is a fundamental protection in international human rights law “that should never be curtailed”.

“Various international bodies have repeatedly called on Israel to end its practice of administrative detention. Such detainees should either be charged with an offence and tried according to international standards, or released immediately,” Zeid said.

READ MORE: What it means to be a Palestinian prisoner in Israel

The open-ended hunger strike is one of the largest in recent years led by the jailed Fatah leader Marwan Barghouti.

The International Committee of the Red Cross (ICRC), which visits Palestinian detainees, urged Israel early this month to allow family visits. Under international law, these “can only be limited for security reasons, on a case by case basis, but never for strictly punitive or disciplinary purposes”, it said.

It added that “Israel detains Palestinians within its territory – but not within the occupied territory as required by the law of occupation”.