Lebanon believes that Saudi Arabia is holding leading Sunni Muslim politician Saad Hariri and plans to work with foreign states to secure his return to the country, a top Lebanese government official told Reuters on Thursday.

Hariri resigned as prime minister on Saturday in a shock declaration read from Saudi Arabia, pitching Lebanon into deep political crisis and pushing the country back to the forefront of a regional struggle between Saudi Arabia and Iran.

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Lebanese President Michel Aoun is seeking help from diplomats to uncover the mystery surrounding Hariri's resignation, al-Manar TV reported on Thursday.

Hariri reportedly met the French Ambassador to Saudi Arabia Thursday in Riyadh, and has met EU, U.K. and U.S. diplomats in recent days.

Meetings with ambassadors aim to ask for help in clarifying the circumstances that led to the resignation of Hariri, who has yet to return to Lebanon, an official source told Reuters.

The sudden nature of his resignation has fuelled widespread speculation in Lebanon that the Saudi-allied politician has been caught up in a regional power struggle between Iran and Saudi Arabia and coerced into stepping down.

Saudi Arabia and Hariri aides have denied reports that he is under house arrest, but he has put out no statements himself denying his movements are being restricted.

Saudi Arabia says the Iran-backed group Hezbollah had "hijacked" the political system.

In his resignation speech, Hariri attacked Iran and Hezbollah for sowing strife in Arab states and said he feared assassination.

"Lebanon is heading towards asking foreign and Arab states to put pressure on Saudi to release Prime Minister Saad al-Hariri," said the official, who declined to be identified because the government had yet to declare the initiative.

The official said Hariri was still Lebanon's prime minister.

"Keeping Hariri with restricted freedom in Riyadh is an attack on Lebanese sovereignty. Our dignity is his dignity. We will work with (foreign) states to return him to Beirut."

Coinciding with a major anti-corruption purge of top Saudis, Hariri's shock announcement has given rise to suggestions from Hezbollah and others that his Saudi business interests had embroiled him in the probe and he was forced to resigning.

"The Saudis appear to have decided that the best way to confront Iran is to start in Lebanon," a European diplomat said.