The FBI launched an investigation into whether Donald Trump had been working on behalf of Russia against American interests – and Trump went to extraordinary lengths to conceal from his own administration the details of his conversations with Russian president Vladimir Putin, according to two bombshell reports.

The New York Times reported on Friday that law enforcement officials were so concerned about Trump’s behavior after he fired James Comey as FBI director that they launched a counterintelligence investigation into whether he was acting as a Russian agent, either intentionally or unwittingly.

According to another report by the Washington Post, Trump has taken unusual steps to conceal the contents of his discussions with Putin. After meeting with the Russian president in Hamburg in 2017, the Post reported, Trump took his interpreter’s notes and instructed him not to disclose what was discussed to other US officials.

On Saturday night, Trump was asked by a Fox News host whether he had ever worked for Russia.

“I think it’s the most insulting thing I’ve ever been asked,” he said.

Why is he so chummy with Vladimir Putin?

He did not give a yes or no answer.

As for his conversations with Putin, he said: “I’m not keeping anything under wraps, I couldn’t care less.”

On Sunday, Democrats said the latest revelations raise serious questions about Trump’s relationship with Putin and Russia.

“Why is he so chummy with Vladimir Putin – this man who is a former KGB agent, never been a friend to the United States, invaded our allies, threatens us around the world, and tries his damndest to undermine our elections?” Senator Dick Durbin, an Illinois Democrat, said on ABC’s This Week.

“Why is this President Trump’s best buddy? I don’t get it.”

Mark Warner of Virginia, the top Democrat on the Senate intelligence committee, said it was suspicious that Trump has “parroted” the policies of Putin.

“I do think it’s curious that throughout that whole summer when these investigations started, you have Vladimir Putin policies almost being parroted by Donald Trump,” he said on CNN’s State of the Union.

This is not a traditional president. He has unorthodox means

“You had Trump say only nice things about Putin – he never spoke ill about Russia. The Republican campaign doctrines softened on Russia and decreased their willingness to defend Ukraine.”

Warner said the US government still does not know what took place in Trump’s meetings with Putin, including another in Helsinki last summer where Trump appeared to embrace Putin’s claim, rejected by US intelligence, that his country had nothing to do with an interference effort in the 2016 election.

“The American government does not know what was discussed between Trump and Vladimir Putin in that frankly pathetic, embarrassing encounter,” Warner said.

Republicans defended the president, saying the US during his administration has imposed tough sanctions against Russia in response to its interference campaign during the 2016 election and its aggression in Ukraine.

“We’ve been very tough on Russia,” House Republican leader Kevin McCarthy said on CBS’s Face the Nation. “Look at the sanctions that we have taken with this administration. I know this administration and I know this Congress is very tough on Russia and we will continue to be so. But I want this president to be able to build a relationship, even on a person level, with all the world leaders.”

Trump meets Vladimir Putin in Helsinki, last July. Photograph: Kevin Lamarque/Reuters

Ron Johnson, a Wisconsin Republican senator and chair of the homeland security committee, said he had only heard “innuendo” about Trump’s interactions with Russia, not any evidence of improprieties. He said there were legitimate reasons to want to guard the president’s conversations with Putin.

“This is not a traditional president,” he told CNN. “He has unorthodox means, but he is president of the United States. It is pretty much up to him in terms of who he wants to read into his conversations with world leaders. He was burned by leaks in other areas and he was pretty frustrated.”

Senator Lindsey Graham of South Carolina, a close ally of the president, was more forceful, telling Fox News Sunday: “I am going to ask the FBI director: ‘Was there a counterintelligence investigation opened up regarding the president as being a potential agent of the Russians?’ I find it astonishing.

“If this really did happen, Congress needs to know about it. How could the FBI do that? What kinds of checks and balances are there?”

Secretary of State Mike Pompeo did not answer specific questions about whether he was aware of the FBI counterintelligence work when he directed the CIA.