Former special counsel Ken Starr said on Sunday that there could be grounds for impeachment if President Trump was lying when he denied reports that he tried to fire special counsel Robert Mueller.

"I think lying to the American people is a serious issue that has to be explored. I take lying to the American people very, very seriously, so absolutely," Starr told ABC's "This Week," when asked if Trump lying about the report would be grounds for impeachment.

The New York Times reported on Thursday that Trump had asked White House counsel Don McGahn to fire Mueller in June. He backed off the order, however, after McGahn threatened to quit, according to the Times.

Trump reportedly claimed that Mueller had conflicts of interest in the probe into alleged collusion between the Trump campaign and Russia.

The president has denied the Times's report, calling it "fake news."

Trump's lawyer handling the Russia probe, Ty Cobb, declined to comment on the matter.

"We decline to comment out of respect for the Office of the Special Counsel and its process," Cobb said.

Various news outlets, including Fox News, which is one of Trump's favored news sources, confirmed the report.

Trump said earlier this week that he looks forward to sitting down with Mueller.

"There's been no collusion whatsoever. There's no obstruction whatsoever, and I'm looking forward to it," he said.