Russian President Vladimir Putin said on Wednesday that North Korea was taking a lot of steps towards denuclearising the Korean Peninsula but Washington was not responding and was making endless demands for full disarmament instead. 

Washington and Pyongyang have been discussing North Korea's nuclear programme since their leaders met in Singapore in June, though that summit's outcome was criticised for being short on details about how and whether North Korean leader Kim Jong Un was willing to give up weapons that threaten the United States. 

Putin, speaking at an economic forum in the Russian port city of Vladivostok, said Pyongyang had taken positive steps and was now waiting for a response. He said it was counter-productive if one side did a lot and the other did nothing. 

"If North Korea does something towards denuclearisation it expects reciprocal steps and not endless demands for full disarmament," said Putin. 

Putin said it was also important that North Korea received international guarantees about its own security and said Kim Jong Un was welcome to visit Russia at any time that was convenient for him. 

The White House said on Monday that U.S. President Donald Trump had received a letter from Kim Jong Un asking for a second meeting.