An Israeli civilian is being held against his will by the Islamist group Hamas in the Gaza Strip, Israel said on Thursday.
Hamas, the Palestinian enclave's dominant faction, declined to comment.
Israel suspects militants in Gaza would use its citizens to bargain for the release of Palestinians held in Israeli jails.
Israel's Defence Ministry said there was "credible intelligence" that Avraham Mangisto, 26, a resident of Ashkelon, the nearest Israeli city to the Gaza Strip, "is being held against his will by Hamas in Gaza". It said he had crossed the border on foot on Sept. 7, 2014.
The statement also said: "The defence establishment is also dealing with an additional case of an Israeli Arab who is being held in Gaza." It did not give details.
Reporting the cases had been barred until Thursday.
Mangisto's brother, Ilan, speaking on behalf of the family said they were placing their trust in the Israeli government to bring Avraham, who was unwell, home.
"This is a distressing humanitarian case because my brother is not well… I ask the Israeli government to bring him home safely and call on the international community to intervene and to bring its influence to bear for his release."
In an emotional plea made outside the family's run-down Ashkelon apartment, Ilan Mangisto also called on Hamas to consider his brother's medical state and to "release him immediately".
Israel's president, Reuven Rivlin, said the family had feared for Mangisto's safety before he crossed to Gaza, adding he should not be considered an asset.
"Without discussing (medical) details, his family was very worried about him before he left … he cannot be considered to hold tactical or strategic value in the struggle between us and Hamas or any other foes," Rivlin told reporters.
Israeli media said Mangisto, who is of Ethiopian origin, crossed of his own volition into the Gaza Strip, circumventing the border along the beach.
Gilad Shalit, an Israeli soldier who was abducted by militants in a cross-border raid in 2006, was released after more than five years in captivity in exchange for over 1,000 Palestinians in Israeli jails.
Israeli Defence Minister Moshe Yaalon said this week Israel was seeking the return of the remains of two soldiers killed during last year's Gaza war.
The Defence Ministry said Israel had "appealed to international and regional interlocutors to demand (Mangisto's) immediate release and verify his well-being" and would "continue to pursue the release and return of the citizen to Israel".
In the 50-day war that ended last August, more than 2,100 Palestinians were killed, mostly civilians. Israel put the number of its dead at 67 soldiers and six civilians.
Israeli air strikes and shelling hammered the densely populated Gaza Strip causing widespread destruction of homes and schools. Hamas and other militant groups launched thousands of rockets and mortar bombs out of the enclave into Israel.
