U.S. Republican Senator Bob Corker stepped up his public feud with Donald Trump on Friday, saying the U.S. president's undermining of his top diplomat was like castrating him in public.

Corker told the Washington Post in an interview that Trump had undercut Secretary of State Rex Tillerson's efforts to enlist China in reining in North Korea's nuclear program by denigrating the diplomat.

"You cannot publicly castrate your own secretary of state" without limiting the options for dealing with North Korea, Corker, chairman of the Senate Foreign Relations committee, told the Post.

Tillerson told a news conference in Beijing two weeks ago that the United States was directly communicating with North Korea on its nuclear and missile programs but it had shown no interest in dialogue.

Trump took to Twitter the next day, saying Tillerson was "wasting his time" trying to negotiate with North Korean leader Kim Jong Un.

"The greatest diplomatic activities we have are with China, and the most important, and they have come a long, long way, Corker said.

"Some of the things we are talking about are phenomenal.

"When you jack the legs out from under your chief diplomat, you cause all that to fall apart, Corker told the Post, adding that working with China was the key to reaching a peaceful settlement with North Korea.

"When you publicly castrate your secretary of state, you take that off the table.