UNITED NATIONS, July 7 (Reuters) - Iran called on the United Nations on Monday to help secure the release of four Iranians kidnapped in Lebanon more than quarter of a century ago, whom Tehran says are being held in Israel.

Israel has denied holding the four -- three diplomats and a journalist -- who were seized north of Beirut by Lebanese Christian militiamen during Israel's 1982 invasion of Lebanon. They were traveling by car to the Lebanese capital from Syria.

Christian warlords have said they were abducted by the Lebanese Forces militia then allied with Israel and killed shortly afterward. Israel has said they were killed by the militia.

But in a letter to U.N. Secretary-General Ban Ki-moon, Iran's U.N. Ambassador Mohammad Khazaee repeated Tehran's long-standing assertion that they were handed over to Israel and remain imprisoned there.

"No credible piece of evidence has ever been presented to indicate the martyrdom of the said Iranian officials to date," said the letter, written two days after the 26th anniversary of the abduction and obtained by Reuters.

It urged Ban and the Security Council "to live up to their responsibilities under the U.N. charter without further delay by reacting to the illegal abduction and continued unlawful detention of Iranian diplomats and journalist by the Israeli regime, and by helping secure their safe and immediate release."

Last week, Lebanon's Iranian-backed Hezbollah group said that, as part of a U.N.-mediated prisoner swap with Israel it expected to take place in mid-July, a German negotiator would receive a report from Israel on the fate of the four Iranians.

The four are Seyyed Mohsen Mousavi, then charge d'affaires of the Iranian Embassy in Beirut, military attache Ahmad Motevaselian, embassy technician Taghi Rastegar Moghadam and journalist Kazem Akhavan of the official news agency IRNA. (Reporting by Patrick Worsnip; Editing by Cynthia Osterman)

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