NEW YORK - Special Counsel Robert Mueller, in a meeting with US President Donald Trump's lawyers in March, raised the possibility of issuing a subpoena for Trump if he declines to talk to investigators in the Russia probe, a former lawyer for the president said on Tuesday.

John Dowd told Reuters that Mueller mentioned the possibility of a subpoena in the early March meeting. Mueller's subpoena warning was first reported by the Washington Post, which cited four people familiar with the encounter.

"This isn't some game. You are screwing with the work of the president of the United States," Dowd said he told the investigators, who are probing possible collusion between the Trump campaign and Russia.

The Post said Mueller had raised the possibility of a subpoena after Trump's lawyers said the president had no obligation to talk with federal investigators involved in the probe.

After the March meeting, Mueller's team agreed to provide the president's lawyers with more specific information about the subjects they wished to ask Trump, the Post reported.

That list, first reported by the New York Times on Monday, includes questions on Trump's ties to Russia and others to determine whether the president may have unlawfully tried to obstruct the investigation.

Russia has denied interfering in the 2016 US presidential election, as US intelligence agencies allege, and Trump has denied there was any collusion between his campaign and Moscow.

Dowd's comments come more than a month after he resigned from the legal team, and they provide a new window into the nature of the Trump lawyers' interactions with Mueller, whom the president has increasingly tried to undermine through public attacks.

On Tuesday, Trump tweeted: "Oh, I see ... you have a made up, phony crime, Collusion, that never existed, and an investigation begun with illegally leaked classified information. Nice!"

A person familiar with the matter, who insisted on anonymity to discuss ongoing negotiations, told the Associated Press that the president's lawyers extrapolated the list of expected questions based off conversations with Mueller's team about the topics prosecutors wanted to cover in a potential sit down with Trump. The questions reflected what the defense lawyers anticipated Trump would be asked, rather than verbatim queries that Mueller's team provided, the person said.

According to the list, the questions range from Trump's motivations for firing FBI Director James Comey a year ago to contacts Trump's campaign had with Russians.

Although Mueller's team has indicated to Trump's lawyers that he's not considered a target, investigators remain interested in whether the president's actions constitute obstruction of justice and want to interview him about several episodes in office. They have not yet made a decision about an interview.

A spokesman for Mueller declined to comment.

Reuters - AP