China's efforts on North Korea not worked out: Trump

China's efforts to rein in North Korea have "not worked" out, US President Donald Trump has said but appreciated Beijing for its attempts to pursue its closest ally to abandon its nuclear ambitions.

"While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out. At least I know China tried!," Trump said in a tweet which came hours before the US and China were set to meet in Washington to talk about North Korea.

The meeting comes amid concern that North Korea is preparing a sixth nuclear test.

There was speculation earlier this year that North Korea was set to provoke Trump with its sixth nuclear test since 2006. However, the reclusive state has instead carried out a series of missile launches.

In a report, The New York Times said that Trump's tweet is "an extraordinary admission of failure in his strategy" for dealing with the rogue regime of Kim Jong-un.

White House press secretary Sean Spice said the US has been very forceful in its political and economic pressure that has been applied on North Korea.

"I think we'll continue to apply that. Obviously, China has played and can continue to play a greater role in helping to resolve this situation, and we will continue to hopefully build on the relationship and the dialogue that we've had with China," Spicer said.

"I think there have been some positive steps that they've taken, both at the UN and economically, to help strengthen the case against North Korea. But I'm not going to get ahead of where we may or may not go. And obviously, we hope that those discussions with China are productive and continue to move us forward," he said.

Chinese President Xi Jinping met with Trump at his private Florida estate in April, a meeting that both leaders said warmed relations between the countries.