China has not done enough on the issue of North Korea for the US, President Donald Trump has said but appreciated the "great chemistry" he shares with Chinese President Xi Jinping. North Korea last year increased the pace of its missile programme. Since February, Pyongyang has fired off 23 missiles. On November 29, the North Korean leader said that his country had achieved full nuclear statehood after what he said was the successful test of a new missile capable of striking anywhere in the United States. Fears of a catastrophic conflict between the US and North Korea spiked as the leaders of the two nations taunted each other, with President Donald Trump calling North Korean leader Kim Jong-un 'Rocket Man'. Trump has called on China to do more to resolve the North Korean nuclear issue. "Not helpful enough, but they have been very helpful. Let's put it this way, they have done more for me than they ever have for any American president. They still haven\u2019t done enough," Trump was quoted as saying in The Wall Street Journal. "But they have done more for me than they have, by far, for any\u2014I have a very good relationship with Xi. I like him. He likes me. We have a great chemistry together. He has done far more for us than they ever have for any American president. With that being said, it\u2019s not enough. They have to do more," Trump insisted. As a result of Chinese help so far, Trump said he has not been tough on China as he would like to be. "We have been much tougher on China, but not nearly as tough as I would be, but they are helping us a lot with North Korea, he said in response to a question. "And you see in North Korea what's happening with North Korea all of a sudden. China\u2019s been helping us a lot, so you can veer a little bit differently, but for the most part everything I have said I have done," he said in response to a question on his electoral promise of being tough on China. North Korea, he said, is a big problem. "It is a big problem, and they should not have left me with that problem. That should have been a problem that was solved by Obama, or Bush, or Clinton or anybody, because the longer it went, the worse, the more difficult the problem got," he said. Trump also said that he probably has a "very good relationship" with North Korean leader Kim Jong Un but did not confirm or deny if he had spoken to the leader in Pyongyang. "I probably have a very good relationship with Kim Jong Un of North Korea. I have relationships with people, I think you people are surprised," Trump said. "Just to be clear, you haven't spoken to the North Korean leader, I mean when you say a relationship with Korea," he was immediately asked. "I don\u2019t want to comment on it\u2014I don\u2019t want to comment, I\u2019m not saying I have or I haven\u2019t. But I just don\u2019t\u2014," Trump responded. The US president denied that delaying military exercises on the Korean Peninsula for the Olympics sends the wrong message to the North Koreans, that he is in some way bending to them.