KYIV  -  The EU’s new top diplomat Kaja Kallas and head of the European Council Antonio Costa arrived in Kyiv Sunday in a symbolic show of support for Ukraine on their first day in office.  “We came to give a clear message that we stand with Ukraine, and we continue to give our full support,” Costa told media outlets including AFP accompanying them on the trip. The European Union’s new leadership team is keen to demonstrate it remains firm on backing Kyiv at a perilous moment for Ukraine nearly three years into its fight against Russia’s all-out invasion. Questions are swirling around the future of US support once Donald Trump assumes office in January and there are fears he could force Kyiv to make painful concessions in pursuit of a quick peace deal. Meanwhile, tensions have escalated as Russian President Vladimir Putin has threatened to strike government buildings in Kyiv with his new Oreshnik missile after firing it at Ukraine for the first time last month.   The Kremlin leader said the move is a response to Kyiv getting the green light to strike inside Russia with American and British missiles, and he has threatened to hit back against the countries supplying the weaponry. As winter begins Russia has also unleashed devastating barrages against Ukraine’s power grid and on the frontline Kyiv’s fatigued forces are losing ground to Moscow’s grinding offensive. “The situation in Ukraine is very, very grave,” Kallas, a former prime minister of Estonia, said. “But it’s clear that it comes at a very high cost for Russia as well.”