Relatives of an engineer from Gaza who disappeared last month in Ukraine said Thursday that he had been kidnapped by Mossad agents and transferred to an Israeli prison.

The engineer, Derar Abu Sisi, is the operating manager of the only power plant in Gaza, the Palestinian enclave that is controlled by Hamas, the Islamic militant group. He is married to a Ukrainian woman and was in the country applying for citizenship, relatives said.

Israeli officials have not commented on the case, which is subject to a court-imposed gag order in Israel. But an Israeli human rights organization, HaMoked, which assists Palestinian detainees in Israeli prisons, received confirmation this week from the Israel Prison Service that Mr. Abu Sisi has been in detention since Feb. 19 and was currently in Shikma prison near the Israeli coastal town of Ashkelon, just north of Gaza.

A lawyer from Israel’s public defender’s office, Michal Orkaby-Danziger, who is representing Mr. Abu Sisi, was unable to comment because of the gag order.

Mr. Abu Sisi’s relatives heard from an Israeli lawyer about 10 days ago that he was in an Israeli prison, was in good health and had not been charged with a crime.

A spokeswoman for Ukraine’s Interior Ministry said that Mr. Abu Sisi, 42, disappeared “under unknown circumstances” in the early hours of Feb. 19 after boarding a train in the eastern city of Kharkiv that was bound for Kiev, the capital, The Associated Press reported.

Mr. Abu Sisi had traveled to Ukraine with his wife, Veronika, but she had returned to Gaza to look after their six children by the time he disappeared. Relatives said that his wife lost contact with him on Feb. 18 when he did not answer his cellphone. Friends in Ukraine told her by telephone that they had dropped him off at the train station, where he was supposed to board a train. He was planning to meet his brother, who was flying into Kiev’s airport that night. Mr. Abu Sisi’s wife has returned to Ukraine to search for him, his sister, Susan Abu Sisi, said.

In a telephone interview, Ms. Abu Sisi said her brother was not a Hamas activist. She noted that he had been working at the power plant for nearly a decade, long before Hamas took control of Gaza in 2007.

Colleagues at the Gaza power plant said Mr. Abu Sisi was a professional engineer who was respected by all parties, including Hamas. They were speaking on condition of anonymity because they were not authorized to comment on the case.

While the reasons for Mr. Abu Sisi’s detention were unclear, it was widely assumed in Gaza that it was somehow linked to his position at the power plant and the successful efforts of Hamas to reduce the station’s dependency on industrial diesel fuel imported from Israel.

In January, the Hamas authorities said that they had managed to adjust the station’s turbines to run on regular diesel fuel, which is smuggled into Gaza from Egypt, saying that Israel was not letting in sufficient amounts of fuel. Mr. Abu Sisi left Gaza twice last year, for a work conference in Egypt and to perform a pilgrimage to Mecca, according to his relatives.