He took to Twitter to complain he would never call North Korean leader Kim Jong-un "short and fat", after its foreign ministry called him "old".
It was one of a series of remarks he made on social media before volunteering his services over maritime claims in the South China Sea.
"I'm a very good mediator," he said.
China, Vietnam, the Philippines, Taiwan, Malaysia and Brunei all have competing claims to territory in the South China Sea.
