President Trump dismissed his former personal attorney Michael Cohen as someone who is "not very smart" and "lying" in order to "get a reduced sentence," on the day Cohen pleaded guilty to making false statements to Congress.

"Michael Cohen is lying and he's trying to get a reduced sentence for things that have nothing to do with me," the president told reporters as he left the White House for the G-20 summit in Argentina Thursday morning.

Cohen pleaded guilty in federal court Thursday to lying to Congress about a Trump real estate project in Russia. Cohen worked as Mr. Trump's lawyer for years, and his deep knowledge of the president's business dealing makes him a key person for Mueller's investigation.

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Sources tell CBS News chief White House correspondent Major Garrett reports Mr. Trump's legal team was made aware of the guilty plea somewhat in advance. Mr. Trump lashed out about Mueller's investigation on Twitter Thursday morning, but those sources tell Garrett that reaction had more to do with frustrations over other matters in the Russia investigation. Sources also tell Garrett Mueller did pose one question to Mr. Trump about the Russia project proposal. The president's lawyers claim Cohen has his own reasons for lying about the Moscow project.

Mr. Trump — who repeatedly dodged questions from reporters as to when he decided not to build a Trump Tower in Russia — said that even if he had gone through with it, it wouldn't matter.

"Here is the thing, even if [Michael Cohen] was right, it doesn't matter, because I was allowed to do whatever I wanted during the campaign," the president told reporters Thursday morning.

Asked why he would employ someone whom he deemed incompetent for so long, Mr. Trump said Cohen did him "a favor a long time ago."

Mr. Trump spent part of his morning Thursday tweeting his frustrations with Mueller's investigation. The president has frequently described Mueller's investigation as a "witch hunt" and "illegal."

"When will this illegal Joseph McCarthy style Witch Hunt, one that has shattered so many innocent lives, ever end-or will it just go on forever? After wasting more than $40,000,000 (is that possible?), it has proven only one thing-there was NO Collusion with Russia. So Ridiculous!" the president tweeted early Thursday.

Mr. Trump has distanced himself from Cohen, who testified in court in a separate case that the president directed him to carry out what amounted to campaign finance violations. Cohen pleaded guilty to those campaign finance violations over the summer.