The United Nations has expressed concern over the Israeli law that allows authorities to force-feed prisoners on hunger strike.

In a letter sent to journalists on Saturday, UN officials in the West Bank said hunger strike is "a non-violent form of protest used by individuals who have exhausted other forms of protest to highlight the seriousness of their situations".

The legislation, passed by Israeli parliament last month, reflected Israel's concern that hunger strikes by Palestinians in its jails could end in death and trigger waves of protests in the occupied West Bank.

But Israel's Medical Association, which considers force-feeding a form of torture and medically risky, has urged Israeli doctors not to abide by the law.

UN officials from the office of the UN Special Coordinator for the Middle East Peace Process, the Office of the High Commission for Human Rights, and the World Health Organisation called the new law "a cause for concern to those who work to protect the right to health of Palestinians in the occupied Palestinian territory".

Peaceful protests such as hunger strikes by Palestinian prisoners are "a fundamental human right", the officials said.

The UN statement comes as the family of a Palestinian prisoner Mohammad Allan held by Israel said his health is dire after waging a hunger strike for the last 56 days.

His father, Naser Allan, said Israel arrested his son in November 2014 and placed him under administrative detention for two six-month periods. He said his son was imprisoned from 2006-2009 for affiliation with the Palestinian armed group, Islamic Jihad.

The younger Allan is on hunger strike to protest his administrative detention, a controversial measure that allows Israel to detain suspects without charge for long periods.

Allan's father said on Sunday that Israeli authorities are threatening to force-feed his son. Allan has refused medicine or vitamins, only drinking water.

Source: Agencies