Former House Speaker Newt Gingrich said special counsel Robert Mueller's Russia investigation represents "the deep state at its very worst."

His comments came during an interview with Fox News' Sean Hannity.

It also followed a Wall Street Journal report that a grand jury has been impaneled in connection with the investigation.

"The Mueller threat has probably been the most deadly because he has the power of the law, he has the ability to indict people, he has the ability to negotiate and let some people off if they'll testify against other people," Gingrich said.

"I regard the Mueller example as the deep state at its very worst and it worries me a great deal. It fits the whole case that we've seen about the Justice Department at times being out of control.

"So, that worries me a lot."

But Gingrich said despite everything, the Trump system is beginning to work.

"[President Donald Trump] is making progress every week," he said. "He's getting things done. He's beginning to move toward a tax cut bill that will be historic. He's gotten more judges approved than [former President Barack] Obama had at this point.

"Obama and a Democratic Congress. Despite everything, I think the Trump system is gradually beginning to really work."

And he said those on the left will be against Trump no matter what he does.

"If you're the left, you hate Donald J. Trump," Gingrich added. "If you're the left, anything you can do to undermine him is okay."