New York: Senior Republican politicians and former US intelligence officials responded with immediate outrage to Donald Trump's extraordinary press conference in Helsinki - in which he declined to back his own agencies over Russian President Vladimir Putin - by describing his performance as "shameful", "treasonous", "bizarre" and "flat-out wrong".

Even by the standards of Trump's unpredictable and norm-smashing presidency, there was a broad consensus that Trump's joint press conference with Putin represented a genuinely unprecedented moment in American history. The leaders' meeting was held just three days after 12 Russian intelligence officers were indicted for conspiring to influence the 2016 election.

Former CIA director John Brennan was among the most critical, saying that Trump had betrayed his country and should be impeached.

"Donald Trump’s press conference performance in Helsinki rises to & exceeds the threshold of 'high crimes & misdemeanors,' Brennan said on Twitter, referring to the standard for impeachment outlined in the US constitution.