RAMALLAH, WEST BANK May 7 (Reuters) - Israel's Supreme Court turned down on Monday an appeal by two Palestinians, who have been on hunger strike for the past 70 days, to free them from detention without trial.

But in its decision, released by the Justice Ministry, the court said security authorities should consider freeing them for medical reasons.

Thaer Halahla and Bilal Diab, described by the court as active members of the Islamic Jihad militant group, have been refusing food in protest at their "administrative detention", a policy Israel applies in many security-related cases.

Around 1,550 Palestinian prisoners have forsworn food in Israeli jails for three weeks in a protest against the detentions, wide denial of family visits and solitary confinement.

Qaddoura Fares, chairman of the main Palestinian prisoners' organisation, decried the ruling as a "death sentence" and accused the court of being politically motivated.

"The court knows the gravity of their status and health, but it has decided to reject the petition. This court is a tool of the [Israeli intelligence services]," Fares said.

The scope of the hunger strike has posed a new challenge to Israel, which has come under international criticism over detention without trial and could face a violent Palestinian backlash if any of the protesters die.

Looking frail, Halahla and Diab had appealed against their detention on Thursday, appearing in court in wheelchairs. Their lawyers and human rights groups said their lives were in jeopardy, and Diab was taken back to an Israeli hospital after he fainted.

DEMONSTRATIONS

In its ruling, the court said "administrative detention causes unease to every judge" but was a "necessary evil" because Israel is "constantly fighting terror".

But it drew authorities' attention to a law under which a prisoner can be released conditionally "if as a result of his illness, his days are numbered or if keeping him in prison significantly endangers his life".

Many Palestinians have family members in prison or experienced detention first-hand during decades of Israeli occupation in the West Bank and Gaza Strip.

Solidarity tents have been pitched in main squares from Gaza to Ramallah. Demonstrations outside the main Israeli prison in the West Bank are held almost daily, repeatedly descending into confrontations stone-throwing Palestinians and Israeli soldiers who respond with teargas and stun grenades.

"We will teach the Zionist enemy a tough lesson should any of the striking prisoners die," Islamic Jihad leader Mohammed al-Hindi said in Gaza.

Three weeks ago, Israel released hunger striker Khader Adnan, a member of Islamic Jihad, amid concern he would die. He agreed to end his 66-day-long strike in exchange for a promise not to renew his detention.

Palestinian officials and rights groups said prisoners belonging to the mainstream Fatah movement were studying an Israeli offer to satisfy their demands.

Concessions include a measured increase in family visitors to prisoners from Gaza, which is subject to broad Israeli travel restrictions, and an agreement to reduce the number of prisoners in solitary confinement.

Other representatives have said the Fatah committee does not represent the whole movement.