U.S. Secretary of State Rex Tillerson said Sunday that President Donald Trump had instructed him to continue diplomatic efforts to calm rising tensions with North Korea, saying "those diplomatic efforts will continue until the first bomb drops."

Speaking on CNN's State of the Union, Tillerson downplayed messages that Trump had previously posted on Twitter suggesting Tillerson was wasting his time trying to negotiate with "Little Rocket Man," a derogatory nickname Trump has coined for North Korea's leader Kim Jong-un.

Trump "has made it clear to me to continue my diplomatic efforts," Tillerson said.

Tillerson's comments Sunday come amid soaring tensions between the United States and North Korea following a series of weapons tests by Pyongyang and a war of words between the two countries' leaders.

North Korea has conducted a series of nuclear tests in recent weeks and launched two missiles over Japan.

Tillerson talks with China

Tillerson has been in talks with China to enlist its help on getting North Korea to back down.

But Trump's recent Twitter messages appeared to undercut Tillerson's efforts, prompting the Senate Foreign Relations Committee Chairman Bob Corker to complain that Trump was publicly castrating Tillerson and hurting diplomatic talks.

Tillerson downplayed those tweets Sunday, telling CNN that Trump and China's President Xi Jinping have an extremely close relationship and that China understands the U.S. position.

"Rest assured that the Chinese are not confused in any way" about the American policy towards North Korea, he added.

In the same CNN interview, Tillerson denied calling the president "a moron" and insisted he and Trump have a strong working relationship.

'Frank and candid relationship'

"I call the president 'Mr. President.' He and I have a very, very open, frank and candid relationship. We have a very open exchange of views on policy," Tillerson said. "At the end of the day, he makes decisions. I go out and do the best I can to execute those decisions successfully.

"He has assembled a very, I think, unconventional team," Tillerson continued. "He himself is an unconventional president. He's assembled an unconventional cabinet. I'm an unconventional pick for secretary of state."

But Tillerson would not answer repeated questions as to whether he called Trump, as has been reported, "a moron" after a tense meeting at the Pentagon in July during which the national security team stressed to a skeptical president the need for a robust American presence around the globe.

First, Tillerson parried: "I'm not going to deal with that kind of petty stuff."

Then he stonewalled: "As I said, Jake, I'm not playing."

Then he side-stepped: "I'm not making a game out of it."

And then he danced around it: "I'm not dignifying the question with an answer, Jake."

The firestorm around the "moron" comment, which was first reported by NBC, prompted Tillerson to hold a remarkable press conference at the State Department earlier this month during which he pledged loyalty to Trump but did not deny using the word. A State Department spokeswoman later denied that Tillerson said it.

But the reports infuriated Trump, who privately bashed his secretary of state to associates and publicly challenged Tillerson to an IQ test.

"And I can tell you who is going to win," Trump told Forbes magazine. The White House later said he was joking.

With files from The Associated Press