BEIRUT (Reuters) - Saad al-Hariri, who quit as Lebanese Prime Minister in a weekend broadcast from Saudi Arabia, has been held under house arrest in the kingdom, a pro-Hezbollah daily said on Tuesday citing unnamed sources. Hariri s office and Saudi-owned media said he flew to the UAE, a Saudi ally and fellow Gulf monarchy, on Tuesday. Aides to Hariri, Lebanon s most influential Sunni politician and a close Saudi ally, have denied claims that he was detained.  Lebanese newspaper al-Akhbar, which is aligned with the Shi ite militant group and political movement Hezbollah, said Hariri  was placed under house arrest hours after arriving in Riyadh last Friday  and had remained in detention since.  On Monday, Saudi Foreign Minister Adel Jubeir dismissed as  nonsense  allegations that the kingdom forced Hariri to resign, and said he was free to leave at any time. Speculation in Lebanon over Hariri s status continued even after Saudi media showed him meeting with King Salman and reported him leaving for the UAE. Hariri s resignation has thrust Lebanon back onto the frontline of the rivalry between Sunni Saudi Arabia and Shi ite Iran that has also wrought upheaval in Syria, Iraq, Yemen and Bahrain.  The coalition government, which Hariri s shock resignation collapsed, included Iran-backed Hezbollah. His declaration came as Saudi Arabia undertook an anti-corruption purge in which royals, ministers and investors have been arrested as the putative next king tightens his grip on power. In a dramatic escalation of the crisis, Saudi Arabia accused Lebanon on Monday of declaring war because of aggression by Hezbollah. Hezbollah leader Sayyed Hassan Nasrallah accused Riyadh last week of forcing Hariri to step down, and said there were  legitimate questions  over whether he had been detained. The al-Akhbar newspaper said that a Saudi security team had been supervising Hariri, citing unnamed sources close to him. The prime minister, whose family made their fortune in the Saudi construction industry, had very limited access to his phones, it said on Tuesday.    Fouad Siniora, a former prime minister and senior member of Hariri s political party, said Hariri would return to Lebanon.  