Former Obama adviser David Axelrod on Thursday called out Donald Trump Jr. and the Trump administration for its "opposition research" defense of the president's eldest son for agreeing to a meeting last year with a Kremlin-linked lawyer promising dirt that could hurt Hillary Clinton.

"I've been involved in politics, covering it and as a strategist for four years. I've been at the highest levels of presidential campaigns," Axelrod began during a panel on CNN. "This is not standard. This is not normal. This is much different than opposition research."

Both Trump Jr. and the president himself have said described the meeting, which took place in June 2016 between Trump Jr., former Trump campaign manager Paul Manafort, Trump's son-in-law and current adviser Jared Kusher, and Russian attorney Natalia Veselnitskaya, as "opposition research."

Trump Jr. admitted Veselnitskaya offered him information on Clinton, but he claimed the information was not "meaningful." Still, critics have said this meeting is evidence of collusion between the Trump campaign and Russia, which members of the U.S. intelligence community say meddled in the 2016 election to help Trump's chances.

Echoing one Democratic senator, Tim Kaine of Virginia, who floated the possibility of treasonous acts, Axelrod said that what Trump Jr. did "is very close to espionage."

"The thing you should do is not have to the meeting," Axelrod said. "The thing you should do is get in touch with the FBI. And the fact that Donald Trump Jr. didn't, the fact that the senior-most people in the campaign were involved in that meeting, is an alarming fact."