Tel AVIV — Two Israeli citizens are being held by Hamas in the Gaza Strip, Prime Minister Benjamin Netanyahu said on Thursday, a situation that could lead to demands for a prisoner exchange between Israel and the militant group.

Hamas declined to confirm or deny it had the captives.

“We are working for the return of the two Israelis who crossed the border into Gaza,” Netanyahu said in a statement, confirming information that has been restricted by the Israeli military censor since last September.

“We hold Hamas responsible for their safety.”

Israel’s Defense Ministry identified one of those held as Abraham Mentis, 26, an Israeli of Ethiopian origin. The other is an Israeli Arab who was not named.

The ministry said it believed Mentis, a resident of Ashkelon, a city close to the Gaza border, had crossed into the coastal territory on foot on Sept. 7, 2014. It gave no details on how or when the Israeli Arab had entered the territory.

Mengisto’s brother, Ilan, said Avraham was unwell and needed help. In an emotional plea outside the family’s apartment in Ashkelon, he called on Hamas to consider his brother’s medical state and “release him immediately”.

Israel has in the past engaged in prisoner swaps with Hamas, most notably in 2011, when Gilad Shalit, a soldier abducted by militants in a cross-border raid in 2006, was released in exchange for more than 1,000 Palestinians held in Israeli jails.

Dozens of those ex-prisoners have been rounded up by Israel since, outraging Hamas, which signaled that until they were freed again it would refuse to respond to Thursday’s charge that it was holding two Israeli captives.