RAMALLAH, West Bank -- An Israel official says that on the 15th day of a hunger strike, the number of Palestinian prisoners participating in the protest has dropped to 870, from 1,300 last week.

The strike is among the largest such protests in years. The strikers demand better conditions in Israeli prisons.

Support for the strike has gained momentum with West Bank marches and a social media campaign showing celebrities in the Arab world drinking salty water in solidarity.

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The strike catapulted imprisoned organizer Marwan Barghouti back into the spotlight and boosted his national leadership credentials.

Israel Prison Service spokesman Assaf Librati did not elaborate Monday on why over 400 prisoners quit the strike. He says the strikers are held in separate wings, monitored by medical staff. He says several are held in isolation.