A Palestinian woman in her forties is in custody, accused of being recruited by Israeli agents in Tunisia to establish a spy ring comprising Arab workers in Lebanon, the daily Al-Safir said in a front-page story on Tuesday.
A Lebanese security source said a plot of some kind had been uncovered, but could give no further details.
But public prosecutor Adnan Addum said no such case had been referred to the judiciary and "this was media talk".
Israel, which recently stepped up its threats against the leaders of Islamist groups, declined to comment.
Al-Safir said the woman, a resident of Nahr al-Barid refugee camp in northern Lebanon, had recruited an Egyptian man living in Beirut, who had also been arrested along with a lover she had tried unsuccessfully to bring into the ring.
Lebanon has foiled attacks on the
US embassy in Beirut
The woman said she had been asked to liaise between two cells plotting to kill Shaikh Hasan Nasr Allah, the head of Hizb Allah, using explosives and a poison to be applied to clothing, the daily added.
Al-Safir said the woman was arrested after a man she had unsuccessfully approached to join the ring warned Hizb Allah, which hounded Israeli troops out of south Lebanon in 2000 after a 22-year occupation.
In 1992, Israel killed Hizb Allah's then leader Shaikh Abbas al-Musawi, his wife and their six-year-old son in an air strike on his car as he was returning to Beirut from the south.
Security sources in Israel said the Mossad spy agency stepped up its hunt for Islamist leaders abroad in 2002, after al-Qaida blew up an Israeli-owned hotel and tried to shoot down an Israeli airliner in Kenya.
At least two Lebanese who Israeli sources said were linked to al-Qaida and Hizb Allah have since died in booby-trap bombings that local authorities attributed to Mossad.
Lebanon has jailed about 30 people since December over bomb attacks on American-style food outlets, foiled attacks on the fortress-like US embassy and alleged plots to assassinate US ambassador Vincent Battle.