US President Donald Trump complained on Wednesday that Russia was helping North Korea to evade international sanctions, signaling frustration with a country he had hoped to forge friendly relations with after his 2016 election win.

"Russia is not helping us at all with North Korea," Trump said during an Oval Office interview with Reuters. "What China is helping us with, Russia is denting. In other words, Russia is making up for some of what China is doing."

China and Russia both signed onto the latest rounds of United Nations Security Council sanctions against North Korea imposed last year. There was no immediate comment from the Russian embassy in Washington on Trump's remarks.

During a 53-minute interview with a fresh Diet Coke near at hand on his desk, Trump also said he was considering a big "fine" as part of an investigation into China's alleged theft of intellectual property; that he has lost all trust in the chief Democratic Party negotiator on immigration in the Senate; and declined to clear up conflicting reports about his use of the phrase "shithole countries" in a White House meeting, which caused an international outcry.

With North Korea persisting as the major global challenge facing Trump this year, the president cast doubt on whether talks with North Korean leader Kim Jong Un would be useful. In the past he has not ruled out direct talks with Kim.

Meanwhile, South Korea said it will continue high-level talks with North Korea with "clear eyes" amid global warnings that Pyongyang might be playing for time to continue its nuclear-arms programme.

"We have to make the most" of the opportunity, South Korean Foreign Minister Kang Kyung-wha told the BBC.

The two Koreas earlier agreed to march under a "unified Korea" flag at next month's Winter Olympics in the South.