Hanoi (Reuters): US 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 US-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 UN sanctions.

In September Kim described Trump as a “mentally deranged US 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.