"He actually sent me birthday wishes and it was a friendly letter," Trump -- who turned 74 on June 14 -- told reporters in the Oval Office when questioned about the missive.

His comments came a day after North Korean state media quoted Kim as saying he had received a letter of "excellent content" from the US president.

"Appreciating the political judging faculty and extraordinary courage of President Trump, Kim Jong Un said that he would seriously contemplate the interesting content," reported the North's official Korean Central News Agency (KCNA), without giving further details about the letter's content.

The front page of North Korea's official Rodong Sinmun newspaper carried a photo of Kim holding Trump's letter as he read it in his office.

In a statement, the White House confirmed "a letter was sent by President Trump and correspondence between the two leaders has been ongoing."

The exchange of missives between the leaders comes after their second summit in February ended without an agreement on what the North would be willing to give up in exchange for sanctions relief.

The KCNA report came just two days after Kim hosted Chinese President Xi Jinping, who wrapped up a highly symbolic visit to nuclear-armed North Korea on Friday.

Xi is expected to meet Trump later this month in Japan during the G20 summit and analysts say the Chinese president intends to use his trip to the North as a way of signalling to Trump his influence with Kim.