The Palestinian organization Hamas, which rules the Gaza Strip, released a video on Monday that allegedly shows an Israeli who has been held captive since 2014.
"I am the prisoner Avera Mengistu.
How long will I be here?” the man says to the camera in Hebrew.
It was unclear when the recordings were made.
It would be the first sign of life from the man who crossed the border from Israel into the blockaded Gaza Strip in 2014.
According to his family, Mengistu suffers from mental health problems.
The office of Israeli Prime Minister Benjamin Netanyahu said in the evening: "The State of Israel is using all its resources to bring its missing and imprisoned sons back to their homeland." The office did not provide any information on the authenticity of the video.
The prisoner's brother said on Israeli television channel 12 that he could not say for sure whether it was really his brother.
Israel and Hamas have been trying to reach an agreement for years.
It is about the handover of the remains of two soldiers from the 2014 Gaza war and the release of Mengistu and a man who crossed the border into the Gaza Strip in 2015.
Hamas published a video with this man in the summer.
The Israeli Arab was seen in a bed with an oxygen mask.
The recordings caused outrage.
About two million people live in the Gaza Strip under very bad conditions.
Hamas violently seized power there in 2007.
Israel then tightened a blockade of the coastal area, which is supported by Egypt.
Hamas is classified as a terrorist organization by the US, EU and Israel.