KIEV, Ukraine — The Palestinian engineer vanished from a Ukrainian train in the middle of the night.

Now, the United Nations refugee agency confirms his wife's fears that he is being held in prison by the Israeli secret service, most likely after being kidnapped nearly three weeks ago.

Dirar Abu Sisi, 42, went missing "under unknown circumstances" in the early hours of Feb. 19 after boarding a train in the eastern city of Kharkiv bound for the capital Kiev, according to Viktoria Kushnir, a spokeswoman for the Interior Ministry. He was in Ukraine applying for citizenship.

Maksim Butkevych, spokesman for the United Nations High Commissioner for Refugees in Ukraine, said Abu Sisi has been in custody in Israel since shortly after his disappearance. The U.N. agency believes he was abducted and illegally transported by Israeli security forces, perhaps with the aid of Ukrainian counterparts.

"We don't know details of his trip from Ukraine to Israel — let's put it this way," said Butkevych. "But unfortunately, what happened looks like a violent abduction and not a legal extradition or any other legal action on the part of authorities."

Abu Sisi's Ukrainian wife, Veronika, 32, alleges the Israeli secret service Mossad carried out the abduction in order to sabotage a key electric power plant in the Hamas-controlled Gaza Strip where he worked as a senior manager.

"I don't suspect it, I am sure of it," Abu Sisi told the AP in a telephone interview. "My husband was the heart of the only electric station in Gaza, or rather its brain. It's a strategic object and they wanted to disable it."

She denied speculation that her husband may be wanted by Israel as a known Hamas sympathizer, saying he had never engaged in politics or any violent groups.

In Gaza, fellow engineers and neighbors described Abu Sisi as a Hamas supporter, pointing to his senior position. He served as the deputy head of the electric power station and posts are traditionally staffed by Hamas loyalists.

Abu Sisi, a Jordan native and father of six, was in Ukraine trying to apply for citizenship after spending 12 years in the Gaza Strip.

The Israeli group HaMoked, which defends the rights of Palestinians, says Abu Sisi has been held in an Israeli prison since Feb 19. He is being kept in the Shikma detention center outside the coastal city of Ashkelon, the group said.

Israel's prison service said it had no information on the case. The internal security service, the Shin Bet, declined comment, citing a gag order.

Veronika Abu Sisi said her husband told her over the phone about a week after his alleged kidnapping that he was in Israel. His lawyer later told her that he was in custody in an Israeli prison and that he had been kidnapped from Ukraine. Reached by the Associated Press, the lawyer declined comment, citing the gag order.

Veronika Abu Sisi said the family decided to return to Ukraine after life in the Gaza Strip became unsafe for their three daughters and three sons and her husband flew to Ukraine to apply for citizenship in January.

In mid-February, after submitting all the paperwork, he was told by Ukrainian authorities to report to a government agency in Kharkiv, which was processing his case. After he turned up, government workers briefly seized his passport, then gave it back to him, saying it was a routine check, Abu Sisi said.

Relieved, the engineer on Feb. 18 boarded a 10:55 p.m. overnight train to Kiev to meet his brother who was flying in from Amsterdam, but the reunion never took place.

Veronika Abu Sisi said she was told by Ukrainian police officials that two unknown men boarded her husband's train car near the central Ukrainian city of Poltava several hours after departure and escorted him out.

"I am in shock," said Abu Sisi, a violin teacher, who converted to Islam to marry her husband, who had come to Ukraine to study as part of an exchange program. "I don't know what to tell my kids about where their father is — he wasn't killed by a bomb, he disappeared from a train in a democratic country!"

Diaa Hadid contributed to this story from Jerusalem.