A senior Hamas official warned Israel on Friday of dire consequences if any Palestinian prisoners die as a result of an ongoing hunger strike.

Two imprisoned Palestinians, Bilal Diab, 34, and Thaer Halahla, 27, have refused food for 66 days, and over 1,550 Palestinian security detainees have joined their strike since April 17.

Diab made matters worse on Saturday by refusing medical treatment.

Riyad Mansour, the Palestinian envoy to the UN, said that Diab and Halahla are suffering from “life-threatening conditions, including severe weight loss, nerve damage, dehydration, decreased muscle tone and low blood pressure.”

They were reportedly transferred to an Israeli hospital on Friday. In total, 10 Palestinian prisoners were transferred to Israeli hospitals as a result of the hunger strike, according to Mansour.

The 10 men are among 1,500 to 2,500 Palestinian prisoners on hunger strike to demand better conditions and an end to detention without trial.

Although Israeli officials and Palestinians give different numbers of hunger strikers, it is still one of the largest prison protests in years.

It involves a quarter to a half of all Palestinians held in Israeli jails, estimated at some 4,600 people. The reasons for their detentions range from throwing stones to killing civilians in brutal terrorist attacks.

Thousands of Palestinians protested across the West Bank and Gaza on Friday in support of security detainees in Israeli prisons. At one gathering in Gaza City, Hamas leader Khalil Haya said that the hunger strike carried out by over 1,500 Palestinian inmates “isn’t a party” and that some of them could die.

“If that happens, you can expect both the expected and the unexpected from us,” he said, and added that Hamas is prepared to mobilize and fight in order to free its imprisoned compatriots.

Islamic Jihad, the other terrorist organization operating in the Gaza Strip, also threatened to break its truce with Israel should any hunger-strikers die.

Robert Serry, the UN’s Special Coordinator for the Middle East Peace Process, urged Israel on Friday to fulfill its international obligations and “do everything in its power to preserve the health of the prisoners.”

The fasting prisoners demand Israel improve prison conditions, increase access to lawyers and family visits, and end solitary confinement and administrative detention.