U.S. President Donald Trump told South Korea on Saturday he had explained to China's President Xi Jinping America's position on the deployment of an anti-missile defense system to South Korea, according to a statement from South Korea's acting leader.

Trump informed South Korean Prime Minister Hwang Kyo-ahn in a Saturday telephone call of his summit discussion with Xi.

China has opposed the deployment of the U.S.-supplied High Altitude Area Defense (THAAD) in South Korea, arguing it could threaten its security, but South Korea and the United States say it is aimed at countering North Korea's missile threat.

China has increased pressure and imposed restrictions on some South Korean companies doing business in China, which led many in the South to believe Beijing was retaliating against the deployment of the missile system.

"President Trump said he and President Xi held in-depth discussions on the seriousness of North Korea's nuclear problem and how to respond to it, and also said he had conveyed the U.S. position on the THAAD deployment," the statement said.

Trump pressed Xi to do more to curb North Korea's nuclear programme in their summit meeting at Trump's Mar-a-Lago resort in Florida, U.S. Secretary of State Rex Tillerson said.

The U.S. military began deployment of the THAAD system in March, a day after the North test-fired four ballistic missiles.

China has not directly said it is targeting South Korean companies.