Saudi Arabia had detained Prime Minister Saad al-Hariri. Picture: Hussein Malla/AP/ File

Beirut - Lebanese President Michel Aoun said on Wednesday that Saudi Arabia had detained Prime Minister Saad al-Hariri, the first time he had said so publicly, and called it an act of aggression against Lebanon.

"Nothing justifies Hariri's lack of return for 12 days. We therefore consider him detained. This is a violation of the Vienna agreements and human rights law," Aoun said at a meeting with Lebanese journalists and media executives.

"We cannot wait longer and lose time. Affairs of state cannot be stopped," Aoun added on Twitter.

Hariri resigned as prime minister on November 4 in a video broadcast from Saudi Arabia and has yet to return to Lebanon. Aoun has said he will not accept the resignation until Hariri returns to Lebanon and submits it in person.

On Wednesday Hariri repeated he would return home.

"I want to repeat and affirm that I am perfectly fine and I will return, God willing, to dear Lebanon as I promised you, you'll see," he wrote on Twitter.

Aoun had said on Sunday that Hariri's freedom was being restricted in Riyadh, casting doubt over anything that Hariri says. Aoun's comments on Wednesday went further, saying for the first time that Hariri was being held.

Saudi Arabia denies holding Hariri against his will or putting pressure on him to resign. In his first public comments since his resignation, an interview given on Sunday to a television station he owns, Hariri said he was free to travel and planned to return to Lebanon within days.