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Ukraine's president played down heightened warnings of a possible Russian invasion in days, saying he had yet to see convincing evidence, despite the United States warning on Sunday that more Russian troops were approaching the borders of Ukraine and that some airlines had canceled or diverted flights there. said President Joe Biden would speak with Ukrainian President Volodymyr Zelenskyy later today. The Ukrainian leader's repeated statements calling for calm among his people - as Russian forces surround Ukraine from three sides in what Russia insists military exercises have intensified over the weekend until Zelenskyy questions strident warnings from US officials in recent days that Russia may be planning to invade as early as mid-week.                                                                                                                                                                                                                                                                                                                                                                   Read | US President Joe Biden and his Russian counterpart Vladimir Putin held a 62-minute phone call on the crisis in Ukraine.                                                                                                                                                                        The United States has gathered information that Russia is considering as a target date on Wednesday, according to a US official with knowledge of the findings. The official, who was not authorized to speak publicly and only did so on condition of anonymity, did not say how definitive the information was. “We will not give Russia an opportunity to surprise here, throw anything at Ukraine or the world,” Jake Sullivan, the US national security adviser, told CNN on Sunday of the warnings Americans.“We will make sure to present what we see to the world in the most transparent and clear way possible,” he said.                             The United States has largely not made public the evidence it says underpins its more specific warnings about possible Russian planning or timing. The Russians deployed missile, air, naval and special operations forces, as well as supplies to support an invasion. This week, Russia moved six amphibious assault ships to the Black Sea, increasing its ability to land on the coast.