Ukrainian politicians on Saturday expressed alarm after US special representative on Kiev affairs Kurt Volker resigned following a summons by US Congress to answer questions in an impeachment inquiry on President Donald Trump.Volker was appointed in 2017 to take charge of US policy on Ukraine, in particular overseeing critical Washington support to the country as it faces a separatist insurgency backed by Russia.He resigned Friday after being ordered to answer questions in the impeachment investigation, a source familiar with the matter told AFP.Ukrainian ex-president Petro Poroshenko called news of Volker's decision "disturbing" and praised his contribution to "strengthening of our strategic partnership with the United States"."It would be hard to overstate his firmness and strategic vision countering the Russian aggression," Poroshenko wrote on Facebook."With Kurt, we all were feeling more confident in Ukraine."Former foreign minister Pavlo Klimkin said the resignation was "not a political one, but a real loss" for Ukraine."We were not indifferent to him - this is a truly rare case in politics ... It's all so sad," Klimkin said on Facebook.Volker, a former US ambassador to NATO, served in his Ukraine position on a voluntary basis while also working as the director of the McCain Institute.A whistleblower complaint this week accused Trump of pressuring Ukraine's current President Volodymyr Zelensky in a July phone call to supply dirt on former US vice president Joe Biden, the favourite to represent Democrats against Trump in the 2020 presidential election.In a letter released Friday, US lawmakers pointed to a tweet by Rudy Giuliani, Trump's personal lawyer, which showed a message from Volker regarding connecting him with a top adviser to Zelensky.The screened message from July 19 - a few days before Trump's call with Zelensky - shows "Kurt Volker" offering Guiliani to schedule a call together with the adviser, Andrey Yermak.The whistleblower complaint said Volker met with senior Ukraine officials to discuss how to "navigate" Trump's demands of Zelensky.Congressional investigators will also interview former US ambassador to Ukraine Marie Yovanovitch, whom Trump reportedly forced out this year for resisting his efforts to pressure Kiev on his demands regarding Biden.Ukraine's current comedian-turned-president Volodymyr Zelensky, who defeated Poroshenko in April poll, has not commented on Volker's departure.