US President Donald Trump branded his impeachment trial a "hoax" in the Swiss resort of Davos on Tuesday, hours before senators were to start debating a push to remove him from office.

US President Donald Trump arrived in Davos on Tuesday for the annual WEF forum, where he was to give a keynote speech just hours before his impeachment trial kicks into high gear in Washington.

Swedish teen activist Greta Thunberg on Tuesday slammed global inaction on climate change in front of the world's top business leaders, as the annual Davos forum faced up to the perils of global warming while bracing for an address from US President Donald Trump.