The Palestinian ambassador in Ukraine on Thursday denied that a Palestinian man who went missing in Ukraine and is now being held in an Israeli jail was a member of Hamas.

The family of engineer Dirar Abu Sisi, who managed the main power plant in Hamas-ruled Gaza, says he was abducted by Israeli agents on board a train last month and spirited away to Israel.

Israeli Prime Minister Benjamin Netanyahu said on Wednesday that Abu Sisi was a Hamas member who had given "valuable information". [ID:nLDE72T29J]

But Mohammed Al-Assad, the Palestinian envoy in Ukraine, told a news conference in Kiev on Thursday: "He was not a member of any organisation".

Describing Abu Sisi's disappearance as a "terrible act of piracy", Al-Assad urged Ukrainian authorities to put pressure on Israel to ensure his safe return to Ukraine.

Abu Sisi, whose wife is Ukrainian, was in the former Soviet republic to apply for Ukrainian citizenship when he disappeared on about Feb. 18 after boarding a train from the northern city of Kharkiv to the capital Kiev.

Al-Assad said Ukrainian authorities were investigating the circumstances of his disappearance. "At the moment there is no proof that Mossad (Israeli intelligence service) officials seized him, but the fact is that he is there (in Israel).

"We consider his disappearance and re-location ... as an international crime for which someone must bear responsibility," he said.

Addressing questions on Wednesday on a live YouTube forum, Netanyahu defended Israel's detention of Abu Sisi. "Abu Sisi is a Hamas man. He is being held in detention in Israel. It is a legal arrest," he said, replying to a question.

"He has provided valuable information, that is all I can say," Netanyahu added.

Other Israeli officials have said Abu Sisi is in custody but have refused further comment, citing court-issued gag orders. Sources in Gaza have said he was not known to have political affiliations to Hamas.

Smadar Ben-Natan, an Israeli lawyer for Abu Sisi, accused Israel in an interview of trying to concoct charges against her client, linking his detention to efforts to gather intelligence on Hamas and on an Israeli soldier held by the group.

Abu Sisi denies having any connection to the soldier, Gilad Shalit. No mention of the soldier was made in allegations lodged against Abu Sisi in closed-door remand hearings, Ben-Natan told Israel's Army Radio.

The German newspaper Der Spiegel on Tuesday quoted an unnamed source as saying Israel may have suspected Abu Sisi of knowing the whereabouts of Shalit, held since a 2006 cross-border raid.

Netanyahu would not answer any questions about whether Abu Sisi was being questioned or had given any details about Shalit