Hamas says it is working to locate them in a Gaza Strip shattered by weeks of Israeli bombardment.
One Israeli source with knowledge of the issue said Hamas was estimated to be holding 90% of the hostages.
Hamas and Israel were expected to release more hostages and prisoners on Wednesday, the last day of an extended six-day truce in the Gaza war.
Israel has said the truce could be prolonged further, provided Hamas continues to free at least 10 Israeli hostages per day.
Israeli spokesman Eylon Levy said on Wednesday the government held Hamas fully responsible for the abduction of everyone held inside Gaza.
Persons:
Abed Sabah, Islamic Jihad, Khalil Al, Hayya, Israel, Gilad Shalit, Shalit, Eylon Levy, Maayan Lubell, Michael Georgy, Angus MacSwan
Organizations:
Hamas, REUTERS, Islamic, HAMAS, U.S . Central Intelligence Agency, CIA, West Bank, Palestinian Prisoners, Palestinian, Thomson
Locations:
Israel, Palestinian, Beach, Gaza City, GAZA, Gaza, Egypt, Iran, Doha, Qatar, Hamas, Jerusalem