Former CIA chief John Brennan hit back at Donald Trump on Sunday, saying the president should be "ashamed" after he attacked as "political hacks" the intelligence leaders who concluded Russia meddled in the 2016 election.

"Considering the source of the criticism, I consider that criticism a badge of honor," Brennan told CNN's State of the Union, where he appeared alongside former Director of National Intelligence James Clapper.

"I found it particularly reprehensible that on Veterans Day Donald Trump would attack and impugn the integrity and the character of Jim Clapper, who served in uniform for 35 years," said the ex-CIA chief.

"I think it's something Mr. Trump should be ashamed of," he added.