JERUSALEM — The International Committee of the Red Cross on Thursday publicly demanded that Hamas provide proof that an Israeli soldier captured five years ago and held in Gaza is still alive. Hamas promptly rejected the demand.

The soldier, Staff Sgt. Gilad Shalit, was a 19-year-old corporal at the time that he was seized from Israeli territory by Hamas and other armed groups from Gaza in a cross-border raid that killed two other Israeli soldiers. The fifth anniversary of his capture falls on Saturday. He has since been held incommunicado in an unknown location by Hamas, the Islamic militant group that now controls Gaza.

The most recent evidence that he was still alive came in October 2009 when Sergeant Shalit’s captors released a videotape of him talking and holding a Palestinian newspaper published on Sept. 14 of that year. In a deal brokered by German and Egyptian mediators, Israel released 20 female Palestinian prisoners in return.

“The total absence of information concerning Mr. Shalit is completely unacceptable,” Yves Daccord, the director general of the international committee, said in a statement on Thursday, adding that “the Shalit family have the right under international humanitarian law to be in contact with their son.”

The committee said that it had requested access to the Israeli soldier several times over the years but that Hamas had never agreed. Hamas has said in the past that it would not allow access for fear that Sergeant Shalit’s whereabouts might be revealed.

In a separate statement on Thursday, the Red Cross also urged Israel to lift its four-year suspension of family visits for hundreds of detainees from Gaza in Israeli prisons.

Yet in response to Hamas’s rejection of the Red Cross demand, Israel’s prime minister, Benjamin Netanyahu, indicated that Israel would toughen the conditions of Palestinians in Israeli prisons who had been convicted of terrorism.

“I have decided to change Israeli policy toward the terrorists sitting in Israeli prisons,” Mr. Netanyahu said in a speech to the Israeli Presidential Conference in Jerusalem. “We are obligated to honor Israeli law and international law and conventions, but not beyond that.”

He announced that a series of measures would be taken, like ending the practice whereby those convicted of terrorism could use their time in prison to obtain academic degrees. “There will be no more master’s degrees in murder and doctorates of terrorism,” Mr. Netanyahu said.

Hamas is demanding the release of 1,000 Palestinian prisoners in exchange for Sergeant Shalit, including many convicted of deadly terrorist acts. Despite broad public sympathy for the Shalits in Israel, the memories of Palestinian suicide bombings and other attacks have made the price for his freedom a subject of sometimes fierce debate.

Responding to the Red Cross demand on Thursday, Ismail Radwan, a Hamas spokesman in Gaza, said that the Red Cross “should have talked about the suffering of 7,000 Palestinian prisoners inside Israel,” and that Israel was blocking the sergeant’s release by refusing to meet his captors’ demands.