A Democrat congressman familiar with Asian affairs has urged President Donald Trump's administration to present a coherent U.S. policy toward North Korea, citing conflicting approaches to the country between Trump and his chief diplomat.

"I think the president and his administration are still grappling to come up with a coherent and comprehensive foreign policy," Rep. Joaquin Castro of Texas said in a recent interview.

Castro referred to the White House's immediate dismissal of Secretary of State Rex Tillerson's offer for unconditional talks with North Korea as one example.

Tillerson made the overture to Pyongyang during a Washington think tank event on Dec. 12, but backed away from it three days later.

"I'm very concerned because I have yet to see a coherent approach by this administration and a purposeful approach where the purpose is to get North Korea to the table" for credible talks on eliminating its nuclear weapons, Castro said.

Even before the latest rift came to light, Tillerson stressed the importance of attempting dialogue and diplomacy with North Korea, even as Trump undercut him by saying the time for talk is over and that the leadership in Pyongyang understands only the threat of force.

Castro expressed hope that Japanese Prime Minister Shinzo Abe will offer good advice to Trump on ways to address North Korea's nuclear and missile programs, as well as other issues of common interest.

"I think there most certainly is a role for Prime Minister Abe to play, in working with the United States to wrangle in North Korea but also to tamp down the tensions in the region," he said.

Castro, a member of the House Foreign Affairs Committee, serves as a co-chair of the U.S.-Japan Caucus, the U.S. congressional caucus devoted to strengthening ties with Japan.

The 43-year-old congressman said he gives the Republican administration "a lot of credit" for securing a series of U.N. sanctions on North Korea in cooperation with China and Russia.

"I thought that was a diplomatic achievement that was very noteworthy and that the administration should be building upon that for the future, rather than getting into verbal fights over Twitter or in the press, with Kim Jong Un," he said, in reference to the North Korean leader.

Castro hailed the U.S.-Japan relationship, saying it "has been one of the few bright spots compared to President Trump's foreign policy with respect to other parts of the world."

"It appears as though President Trump and Prime Minister Abe have a good working relationship, and so that's been a very positive thing," he said.

As for speculation that Tillerson may step down due to strained ties with Trump over a host of issues such as North Korea, Iran and the Paris climate accord, Castro said that if the two men "continue to offer conflicting paths forward on so many critical issues, I don't know how long that relationship can last."

"And I don't know how long he can stay in that job," Castro said of Tillerson. "The thing is we need somebody in that role who the president trusts completely, who the president is not going to contradict, who is on the same page with the White House."