The International Committee of the Red Cross appealed to Palestinian armed groups in Gaza on Thursday to prove an Israeli soldier captured five years ago is alive.
The agency's director-general, Yves Daccord, called the absence of information concerning Sgt. Gilad Schalit "completely unacceptable."
Gaza's Hamas rulers turned down the Red Cross' previous requests to visit Schalit, who was captured June 25, 2006, by Hamas-linked militants in a cross-border raid.
There has been no sign of life from him for nearly two years, when Hamas released a short videotape.
Hamas spokesman Sami Abu Zuhri accused the Red Cross of being a tool of Israeli intelligence, and said the Schalit case would be ended by trading him for Palestinian prisoners held in Israel.
Israeli Prime Minister Benjamin Netanyahu said Israel would strip Palestinian prisoners of privileges in retaliation for Hamas' rejection of the Red Cross appeal.
Netanyahu did not offer many details, but said, "I stopped that absurd practice whereby terrorists in Israeli prisons who murdered innocent people register for academic studies. ... There won't be M.A. students for murder or doctoral candidates for terror."
In Gaza City on Thursday, about 20 relatives of Palestinian prisoners gathered outside the Red Cross office, throwing eggs and empty bottles at the building and demanding an apology.
The crowd, mostly women, broke a sign on the wall of the building before Hamas police dispersed the protesters.