U.S. President Donald Trump said on Thursday that North Korean leader Kim Jong-un is in "good health." 

Trump made the comment on Kim’s health on Twitter, adding to never underestimate the North Korean leader, without elaborating further. 

The short statement came after letters exchanged between Trump and Kim were revealed in excerpts from an upcoming book by veteran journalist Bob Woodward. 

Among the revelations in the book, Trump reportedly told the journalist that Kim "tells him everything," including a  "graphic account" of how Kim had his own uncle assassinated. 

Trump's tweet also follows months of speculation that Kim may have suffered a health problem, sparked by his long disappearance from the public eye earlier this year.