MOSCOW (Reuters) -President Vladimir Putin said on Thursday that Russia supported a U.S. proposal for a ceasefire in Ukraine in principle, but that any truce would have to address the root causes of the conflict and many crucial details needed to be sorted out. Russia's invasion of Ukraine in February 2022 has left hundreds of thousands of dead and injured, displaced millions of people, reduced towns to rubble and triggered the sharpest confrontation for decades between Moscow and the West. Putin's heavily qualified support for the U.S. ceasefire proposal looked designed to signal goodwill to Washington and open the door to further talks with U.S. President Donald Trump.