US President Donald Trump said negotiating with North Korea over its nuclear program would be a waste of time Sunday after it emerged that Washington had channels of contact with Pyongyang.

"I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man," Trump wrote on Twitter, referring to North Korean leader Kim Jong-Un.

Speaking Saturday after a day of talks with China's President Xi Jinping and top diplomats, Tillerson said that US officials are in touch with Pyongyang despite an escalating war of words between Trump and Kim and the imposition of US-led sanctions.

Asked how he could know whether the North would even contemplate coming to the table, Tillerson told reporters: "We are probing, so stay tuned."

But the State Department later said that North Korea had shown no interest in such discussions and Trump wrote that he regarded any possible talks as futile.

"Save your energy Rex, we'll do what has to be done!"