President Donald Trump on Wednesday said the United States had a "great relationship with China" as he stood beside former Iowa Governor Terry Branstad, the new US ambassador to Beijing.

"We have a great relationship with China and I really like President Xi," Trump said in a speech at an Iowa community college.

The comment came a day after Trump said Chinese efforts to persuade North Korea to rein in its weapons programs had failed.

“While I greatly appreciate the efforts of President Xi and China to help with North Korea, it has not worked out. At least I know China tried!” Trump had written on Twitter.

It remained unclear whether his remark represented a significant shift in his thinking in the US struggle to stop North Korea's nuclear program and its test launching of missiles or a change in US policy toward China.

Trump has held high hopes for greater cooperation from China to exert influence over North Korea, leaning heavily on Chinese President Xi Jinping for his assistance. The two leaders had a high-profile summit in Florida in April and Trump has frequently praised Xi while resisting criticising Chinese trade practices.