A U.S.-mediated ceasefire between Russia and Ukraine appeared under serious strain on Sunday, with both sides trading blame for weekend attacks. The three-day pause, announced on Friday by President Donald Trump, is part of a broader U.S.-led push for peace that has so far failed to end the more than four-year-old war despite months of shuttle diplomacy. Three people were killed in Russian drone strikes on areas near the front line, and more than 200 battlefield clashes had taken place since early Saturday, Ukrainian officials reported on Sunday. They held off explicitly accusing Russia of violating the U.S.-brokered deal, which also included a swap of 1,000 prisoners of war from each side. It was unclear when the exchange would take place. On Sunday, Russia's Defense Ministry accused Ukraine of flouting the ceasefire, saying it had downed 57 Ukrainian drones over the past day and "responded in kind" on the battlefield.…