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 began to "deteriorate 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," he said.

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, 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".

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

The situation in the occupied territories has been particularly tense over the past month as Palestinians engage in marches and protests in solidarity with the prisoners.

Israeli police released three Palestinian guards of the al-Aqsa Mosque compound from detention on Thursday and banned them from entering the premises until June 4.



Tensions flared a day earlier when more than 800 Israeli settlers visited the compound on Israel’s “Jerusalem Day”, an annual celebration on the anniversary of Israel's occupation of East Jerusalem in 1967, an official from the Islamic Waqf, which runs al-Aqsa, told Al Jazeera.

Israeli police had arrested the three guards after assaulting them near the Chain Gate.



Khalil Tarhoni, one of the guards, is currently hospitalised after he was severely beaten by Israeli police in an incident caught on video.



Witnesses told Al Jazeera that Jewish extremists were admitted inside the compound in unusually large numbers,” during the visitation hours designated for non-Muslims.



“Jewish extremists began praying near the Chain Gate and the guards protested. Some Muslim worshippers began chanting Allahu Akbar,” one witness recalled.



Non-Muslim prayer has been banned at the compound for centuries. Since the 1967 occupation, Jews have been allowed to enter the compound under the protection of Israeli forces, but not to pray in the premises.



Sheikh Azzam al-Khatib, head of the Islamic Waqf, told Al Jazeera: “Israeli police assaulted the guards for no reason. Palestinian guards are doing their work. The presence of the Israeli police in itself constitutes an aggression on the Noble Sanctuary.”

READ MORE: Why Jerusalem is not the capital of Israel

Palestinians have long expressed concern that right-wing Jewish groups want to demolish the mosque and build a Jewish temple in its place. Jews, who refer to the site as Temple Mount, believe it is the area where two Biblical temples once stood.



In 1984, members of a Jewish organisation attempted to bomb the site with the hope that the Third Temple would be built on its ruins.

Also, in 1990, Israeli border police killed 22 Palestinians during a demonstration triggered by an attempt led by an Orthodox Jewish group to lay the cornerstone for a new temple in the compound.



Israel - which captured the western half of Jerusalem in the 1948 Arab-Israeli war - frames Jerusalem Day as the "reunification" of the east and west of its capital.



The annual celebration is a day when right-wing, mostly young Israelis rampage around East Jerusalem's Old City, carrying Israeli flags and shouting anti-Palestinian, anti-Arab and anti-Muslim racist slogans.

Large groups often parade through the Muslim Quarter of the old city and provoke residents by banging on Palestinian stores and homes.

Additional reporting by Ibrahim Husseini.