US President Donald Trump has avoided giving a direct answer when asked if he currently is or has ever worked for Russia.

A published report shows federal law enforcement officials were so concerned about Trump’s behaviour after he fired James Comey from the FBI that they began investigating whether he had been working for the US adversary against American interests.

Trump said it was the “most insulting” question he had ever been asked.

Trump responded to the report during an interview broadcast on the Fox News Channel after host Jeanine Pirro asked whether he is currently or has ever worked for Russia.

“I think it’s the most insulting thing I’ve ever been asked,” Trump said. “I think it’s the most insulting article I’ve ever had written, and if you read the article you’ll see that they found absolutely nothing.”

Trump never answered Pirro’s question directly, but went on to say: “If you ask the folks in Russia, I’ve been tougher on Russia than anybody else, any other... probably any other president, period.”

The New York Times reported that FBI agents and some top officials became suspicious of Trump’s ties to Russia during the 2016 presidential campaign but did not open an investigation at that time because they were not sure how to approach such a sensitive and important probe.

But Trump’s behaviour in the days around Comey’s May 2017 firing as FBI director, specifically two instances in which he seemed to tie Comey’s ousting to the Russia investigation, helped trigger the counter-intelligence part of the investigation, according to the newspaper.