Lebanon's president has accused Saudi Arabia of holding hostage Prime Minister Saad al-Hariri along with his family and called it an act of aggression.
"We will not accept him remaining a hostage whose reason for detention we do not know," President Michel Aoun said in a statement.
Mr Hariri's abrupt resignation as premier in a statement televised from the Saudi capital Riyadh on November 4 thrust Lebanon to the front of a Middle East contest for power between Saudi Arabia and Iran.
Mr Aoun has said he will not accept Mr Hariri's resignation until he returns to Lebanon to formally submit it and explain his reasons, which Mr Hariri has said he will do in the coming days. Lebanese politicians close to Mr Hariri said last week that Saudi Arabia had coerced him into quitting.
Mr Hariri has denied being kept in custody by Saudi Arabia and pledged on Wednesday to return soon. Riyadh denies detaining him or forcing him to resign.
Saudi Arabia has long been considered Mr Hariri's main external supporter. Mr Aoun is a political ally of Lebanon's Hezbollah, a powerful Shi'ite Muslim group with close ties to Iran.
Lebanon's coalition government was formed last year through a political deal that made Mr Aoun president, Mr Hariri prime minister, and brought members of Hezbollah into the cabinet.
Saudi Arabia last week accused Lebanon of having declared war on it because of Hezbollah's role in other Arab countries. It regards Hezbollah as a terrorist organisation.
"Nothing justifies Hariri's lack of return for 12 days. We therefore consider him detained," Mr Aoun said.
He added that Lebanon had confirmed that Mr Hariri's family were under detention in their house in Saudi Arabia and were searched whenever they entered or left it.
Mr Hariri wrote on Twitter that he was "perfectly fine" and would return, "God willing, to dear Lebanon as I promised". On Wednesday he said he would return in a couple of days but that his family was staying in Saudi Arabia, calling it "their country".
Mr Hariri holds Saudi nationality and his family fortune is based on the construction company Saudi Oger built by his father Rafik al-Hariri, who served twice as Lebanese prime minister and was assassinated in 2005.