The Trump administration has blocked the US ambassador to the European Union from testifying to a congressional impeachment investigation.

Gordon Sondland was due to meet behind closed doors on Tuesday with staff from three Democratic-led House committees.

But the envoy was directed by the Department of State not to attend.

Mr Sondland was set to be grilled about any role he played in prodding Ukraine to investigate President Donald Trump's political rival Joe Biden.

Mr Trump said Mr Sondland would only have been testifying to a "kangaroo court".

The Democratic-led impeachment inquiry is trying to establish whether the Republican president withheld nearly $400m in aid to nudge Ukraine's president into launching an inquiry into Mr Biden, whose son, Hunter Biden, was on the board of a Ukrainian energy company.