HANOI (Reuters) - U.S. President Donald Trump said in a tweet on Sunday that North Korean leader Kim Jong Un had insulted him by calling him  old  and said he would never call Kim  short and fat.  Trump made the comment after attending the Asia-Pacific Economic Cooperation (APEC) summit in Vietnam. In a series of tweets he also said Chinese President Xi Jinping was  upping sanctions  on North Korea in response to its nuclear and missile programs and that Xi wanted Pyongyang to  denuclearize.  During Trump s visit to Beijing last week Xi reiterated that China would strive for the denuclearization of the Korean peninsula but offered no hint it would change tack on North Korea, with which it fought side by side in the 1950-53 Korean war against U.S.-led forces. One of Trump s tweets read:  Why would Kim Jong-un insult me by calling me  old,  when I would NEVER call him  short and fat?  Oh well, I try so hard to be his friend - and maybe someday that will happen!  Speaking later in Vietnam s capital, Hanoi, Trump said  it would be very, very nice  if he and Kim became friends.  That might be a strange thing to happen but it s a possibility,  he said. Trump has traded insults and threats with Kim in the past amid escalating tension over Pyongyang s nuclear and missile programs as North Korea races toward its goal of developing a nuclear-tipped missile capable of reaching the United States -something Trump has vowed to prevent. North Korea conducted its sixth and most powerful nuclear bomb test on Sept. 3, prompting another round of U.N. sanctions. In September Kim described Trump as a  mentally deranged U.S. dotard  whom he would tame with fire. His comments came after Trump threatened in his maiden United Nations address to  totally destroy  the country of 26 million people if the United States were threatened. After North Korea s Foreign Minister Ri Yong Ho addressed the U.N. General Assembly in September Trump tweeted:  Just heard Foreign Minister of North Korea speak at U.N. If he echoes thoughts of Little Rocket Man, they won t be around much longer!  North Korea has conducted dozens of ballistic missile tests in defiance of U.N. sanctions. It has vowed to never give up its weapons programs, saying they are necessary to counter hostility from the United States and its allies. The United States has said that all options, including military, are on the table, although its preference is for a diplomatic solution. 