DUBAI/RIYADH (Reuters) - French President Emmanuel Macron held hastily scheduled talks in Riyadh on Thursday with Saudi Crown Prince Mohammed bin Salman amid rising tensions between Saudi Arabia and Iran, notably over Lebanon and Yemen. Macron, who flew in from a visit to the United Arab Emirates, had earlier declined to discuss a wave of high-level arrests for corruption in Saudi Arabia, but said it was vital to work with the kingdom for the stability of the region. The first face-to-face talks between the two men focused on regional questions, in particular Yemen and Lebanon, and  ensuring the preservation of stability in the region , the French presidency said in a statement after the meeting. Two top Lebanese government officials said on Thursday that Riyadh was holding Lebanon s Saad al-Hariri captive and a third told Reuters that the Saudi authorities had ordered Hariri to resign while he was in Riyadh last weekend, and put him under house arrest. Saudi Arabia has denied that he is under house arrest, but Hariri himself has not denied that his movements are being restricted. France has close ties with Lebanon, a former colony, and with Hariri, who has a home in France after spending several years in the country. Macron said at an earlier news conference in Dubai there had been informal contacts with Hariri, but no request to transfer him to France.   They discussed the situation in Lebanon following the resignation of Prime Minister Hariri,  the French presidency said.  President Macron reiterated the importance France attaches to Lebanon s stability, security, sovereignty and integrity.  The statement made no mention on whether Macron had spoken with or seen Hariri while in Riyadh. In recent years, France has been able to nurture new links with the Sunni-ruled Gulf Arab states due to its tough stance on Iran in nuclear negotiations, and the broad similarity of their policies on conflicts across the Middle East. However, the 32-year-old Saudi crown prince has emphasized closer ties with U.S. President Donald Trump at a time when Macron has in turn sought to improve relations with Iran, Saudi Arabia s Shi ite rival for regional influence. Macron has said he will travel to Iran in 2018, potentially becoming the first French president to travel there since 1971.  In Yemen, the Saudi-led military coalition fighting the Houthi movement said on Monday it had closed all air, land and sea ports to stem the flow of arms to the Houthis from Iran. The United Nations said the move risked causing  the largest famine the world has seen for many decades, with millions of victims . The French presidency said Macron underlined his concerns regarding the humanitarian situation. On Iran, Macron repeated earlier in Dubai that he wanted to keep the landmark 2015 nuclear deal, which Trump has challenged.  But he said he was  very concerned  by Iran s ballistic missile program, mentioning a missile fired from Yemen and intercepted by Saudi Arabia on Saturday, and raised the prospect of possible sanctions with regard to those activities.  There are extremely strong concerns about Iran. There are negotiations we need to start on Iran s ballistic missiles,  he said.  Like what was done in 2015 for the nuclear activities, it s necessary to put a framework in place for Iran s ballistic activities and open a process, with sanctions if needed, of negotiation that would enable (that).  Iran has denied providing ballistic missiles to Yemeni Houthi rebels and says its missile program is purely defensive and should not be linked to the nuclear deal. Macron reaffirmed his intention to go to Iran as part of efforts to talk to all the actors in the region, but warned about decisions that could destabilize the region further.   We need to have a truthful dialogue,  he said.  It s not about showing any naivety toward Iran, it is about standing beside our allies, in particular the United Arab Emirates, but it is also about not having a diehard policy that could create imbalances or even conflicts in the region.  