U.S. President Barack Obama and Russian President Vladimir Putin discussed the Syria crisis and the situation in Ukraine during a meeting Monday on the sidelines of the climate summit in Paris, a White House official said.Obama stressed the importance of targeting Daesh militants in Syria and not focusing military attacks against rebel groups who oppose Syria's Bashar al-Assad, the official said on customary condition of anonymity."The two presidents discussed the imperative of making progress on the Vienna process to bring about a ceasefire and political resolution to the civil war in Syria," the official said, referring to international talks in the Austrian capital on the Syria crisis.Obama told Putin he believes Assad must leave power as part of a that transition, and both leaders said their foreign ministers will continue to work on the diplomatic process, the official added.Obama expressed regret over the recent death of a Russian pilot after Turkey shot down a Russian warplane at its border with Syria, the official said. The plane downing triggered Moscow's deployment of an advanced missile system to Syria . Obama encouraged de-escalation between Russia and Turkey, the official said.During the discussion, the American president also emphasized a diplomatic solution to the Ukraine crisis, adding that sanctions against Russia can be rolled back when Moscow honors the Minsk cease-fire accord, according to the White House official, who spoke on background.The two leaders met for 30 minutes alongside the climate talks in Paris, the Kremlin said separately. Dmitry Peskov, the spokesman, told reporters that Obama had expressed regret at the meeting over the downing of a Russian plane by Turkish military jets, and that both sides had spoken in favour of moving towards a political settlement of the Syria crisis.Later in the day, U.S. Ambassador to NATO said U.S. data on Russian incursion into Turkish airspace backs Turkey's explanation.Meanwhile, Russian Su-34 fighter bombers flew in Syria for the first time with air-to-air missiles for self-defense on Monday, a Russian air force official told Russian news agencies, less than a week after a Turkish F-16 shot down a Russian jet.Igor Klimov, the official, said the air-to-air missiles were capable of hitting targets at a distance of up to 60 kilometers (37.28 miles).