Law enforcement officials were so concerned by President Donald Trump’s behaviour after he fired FBI Director James Comey that they launched a probe to determine if was working for Russia against US interests.

The New York Times report cites unnamed former law enforcement officials and others familiar with the investigation.

The inquiry forced counterintelligence investigators to evaluate if Mr Trump was a potential threat to national security, and they also sought to determine whether Mr Trump was deliberately working for Russia or had unintentionally been influenced by Moscow.

The Times reports that FBI agents and some top officials became suspicious of Mr Trump’s ties to Russia during the 2016 presidential campaign but didn’t launch an investigation at that time because they weren’t sure how to approach such a sensitive and important probe, according to the sources.

But Mr Trump’s behaviour in the days around Mr Comey’s May 2017 firing, specifically two instances in which he seemed to tie Mr Comey’s ousting to the Russia investigation, helped trigger the counterintelligence part of the investigation, according to the Times‘ sources.

Robert Mueller took over the investigation when he was appointed special counsel days after Mr Comey’s firing.

The overall investigation is looking into Russian election interference and whether Mr Trump’s campaign coordinated with the Russians.

The Times says it’s unclear whether Mr Mueller is still pursuing the counterintelligence angle.

Mr Trump’s lawyer,  Rudy Giuliani, told the Times he had no knowledge of the inquiry but said that since it was opened a year and a half ago and they hadn’t heard anything, apparently “they found nothing”.

Mr Trump has also repeatedly and vociferously denied collusion with the Russians.

-with AAP