US leader Donald Trump said on Thursday that the United States will cut its tariffs on Chinese goods related to fentanyl to 10 percent from 20 percent, after his summit with President Xi Jinping in South Korea.

Xi will work "very hard to stop the flow" of fentanyl, the US president said after departing from Gimhae Air Base in Busan aboard Air Force One at the end of their one-hour-and-forty-minute talk, and the tariff was reduced "because I believe they are really taking strong action."

Trump said a great deal of decisions were made in his summit with Xi Jinping, describing it as "an amazing meeting" and a "great success".

China will purchase "tremendous amounts" of US soybeans and other farm products "starting immediately", and a one-year agreement on rare earths has been reached that will be extended after a year, he said.

"All the rare earths has been settled, and that's for the world," Trump said.

"They're not going to impose the rare earth controls.

"There's no roadblock at all on rare earths – that will hopefully disappear from our vocabulary for a little while."

Trump has said he expects to reduce tariffs on Chinese goods in exchange for Beijing's commitment to curb the flow of precursor chemicals to make fentanyl, a deadly synthetic opioid that is the leading cause of American overdose deaths.

Beijing has sought the lifting of 20 percent tariffs over fentanyl among other matters. The reduction to 10 percent brings the total combined tariff rate on China down from 57 percent to 47 percent.

“I guess on the scale from 0 to 10, with ten being the best, I would say the meeting was a 12,” Trump said. “I think it was a 12.”

Trump said he would go to China in April and Xi would come to the United States “some time after that.” The president said they also discussed the export of more advanced computer chips to China, saying that Nvidia would be in talks with Chinese officials.

Trump said he could sign a trade deal with China “pretty soon.”

“We have not too many major stumbling blocks,” Trump said.

Trump, who was speaking on board Air Force One departing South Korea on the last leg of his Asia trip, also said he had a "nice" talk with Canada's leader Mark Carney after a new tariff row erupted between the neighbours.

"We had a very nice conversation with him," Trump said, almost a week after announcing he would be hiking tariffs on Canadian goods by 10 percent more and terminating all trade talks following what he called a "fake" anti-tariff ad campaign that featured the late ex-president Ronald Reagan.

On North Korea, he said he had been too "busy" to meet its leader, Kim Jong-un, during his visit this week to South Korea but added he could return. (Agencies)