US President Donald Trump urged Chinese President Xi Jinping to take action on North Korea as the two conducted talks on the sidelines of the Group of 20 summit Saturday.

Trump called North Korea's nuclear and missile programmes a "substantial problem" and said "something has to be done about" them.

The US has been urging China to isolate North Korea, but in recent weeks has expressed dissatisfaction with Beijing's efforts to date.

Trump acknowledged it could take time to come to an agreement that ends North Korea's nuclear programme.

"It may take longer than I'd like, it may take longer than you'd like, but there will be success in the end one way or the other," he said.

In his remarks before the talks, Xi did not directly address North Korea, but acknowledged "differences" and the need to work broadly together with the United States.

"We'd like to work with the US, and steadily stay in the right direction to develop the China and US relationship," he said in Chinese.

"We should respect each other mutually, benefit each other mutually, and explore the areas of cooperation.

"The leaders also discussed trade issues, a substantial sticking point between Trump and other G20 leaders.

US trade with China must be "reciprocal," Trump said.

White House officials told reporters aboard Air Force One after the summit that the talks were "direct" on both North Korea and economic issues, with Trump telling Xi he expects progress.

- dpa