Ashkelon (Israel) (AFP) - Two Israelis are being held captive by Hamas in the Gaza Strip after having crossed into the Palestinian territory, Israeli officials said Thursday, raising fears of another hostage crisis.

In 2011, Israel released more than 1,000 Palestinian prisoners in exchange for soldier Gilad Shalit, who had been held by Hamas for five years, and critics said that would encourage further abductions.

And the Shalit case already appears to be having a bearing on this one, with a Hamas official saying former prisoners re-arrested by Israel last year must be freed before there is any talk about releasing the Israelis.

The defence ministry said that, "according to credible intelligence" Avraham Mengistu, an Israeli of Ethiopian descent, "is being held against his will by Hamas in Gaza".

Mengistu had "independently crossed the security fence" into Gaza, the Palestinian territory ruled by Hamas, a statement said.

The defence ministry said Mengistu entered the Palestinian territory on September 7, shortly after last summer's war between Israel and Hamas.

"The defence establishment is currently dealing with an additional case of an Israeli Arab also being held in Gaza," it said, without elaborating.

Israeli Defence Minister Moshe Yaalon said the two were "held by Hamas," and Prime Minister Benjamin Netanyahu said: "We hold Hamas responsible for their welfare."

"I expect the international community, which expresses its concern over the humanitarian situation in Gaza, to issue a clear call to release the citizens and ensure their return," Netanyahu said in a statement, adding that Israel was "working to return the two," without elaborating.

Israel does not allow its citizens to enter Gaza, partly out of fears that they may be used as bargaining chips to demand concessions, including the release of prisoners.

The case had been kept quiet due to an Israeli gag order, which was lifted on Thursday.

Media reports said information on the Israeli Arab was still under a gag order, but implied he had been missing for a number of months.

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- Family demands return -

Mengistu's family briefly addressed journalists outside their home Thursday, calling on Hamas to release him and the Israeli government to work toward his safe return.

"The family chose until now to remain discreet," said brother Gasho Mengistu, surrounded by other family members, including his mother, who had tears in her eyes.

Gasho Mengistu said his brother was not in good health but did not provide details.

A friend of the family, wearing a shirt with Mengistu's name on it, told reporters outside their home that many questions remained unanswered. The family was told earlier on that they should keep quiet, he said.

Avi Yaalo said "we know that he has been in Gaza for several months. We don't have any more information. The family is demanding the return of their son in good health."

Members of Israel's 135,000-strong Ethiopian Jewish community say they suffer from racism and discrimination. They have staged several rallies against alleged police brutality and racism in recent months.

Lawmaker and former head of the Shin Bet security service Avi Dichter told army radio it could be possible to combine indirect talks on their cases with ongoing efforts to secure the return of body parts of two Israel soldiers killed during the 2014 Gaza war.

"If within the parameters of negotiations to get back the bodies of our soldiers... we can manage to get these two Israelis who are in Gaza it would be a great achievement," said Dichter, of Netanyahu's Likud party.

The army has identified the dead soldiers as Staff Sergeant Oron Shaul and Lieutenant Hadar Goldin.

A member of the Hamas political bureau in Gaza told AFP the matter was in the hands of the group's armed wing.

However, he said "nothing is for free," speaking on condition of anonymity. "In advance of any discussion Hamas demands the release of all the prisoners freed in the swap for Gilad Shalit and subsequently re-arrested."

Some of those arrests contributed to a heightening of tensions between Israel and Hamas last summer that set off a chain of events that led to the 50-day war.