Donald Trump ordered almost $400m in aid to Ukraine to be frozen days before he allegedly asked its president to investigate the son of Joe Biden, US media said late Monday, quoting administration officials. The reports in the Washington Post and New York Times add to a burgeoning crisis around Trump's July phone call with Volodymyr Zelensky and a possible attempt to coerce him into digging up information about Biden's son's business dealings in Ukraine. Members of the Trump administration were instructed to tell lawmakers the freeze was due to an "interagency process", but to provide no additional information, the Post said. Several Democrats argue that Trump's call for Ukraine to investigate Biden -- and what they suspect was a threat to condition the aid to Ukraine on the country doing so -- is impeachable conduct.