Kim asked for a fourth meeting with Trump in a letter in late August, sources said

Followed the president reporting a 'beautiful letter' from Kim earlier last month 

The pair have met thrice since June 2018, including two formal summits

Those talks were not substantive and missile testing has continued by the North

Kim Jong-un invited Donald Trump to visit Pyongyang in a letter outlining his 'willingness' to meet the President after dunuclearisation talks stalled, sources said.

Kim, in the letter sent in late August, extended an invitation for Trump to visit the North Korean capital, the Joongang Ilbo newspaper reported, citing an unidentified diplomats. 

Earlier last month, Trump said he had received a 'very beautiful letter' from Kim, but U.S. officials have not said anything about a second letter in August.