U.S. President Donald Trump said on Monday that he exchanged "very friendly" letters with North Korean leader Kim Jong-un.

Trump told reporters at the Oval Office that he sent Kim the friendly letter in response to a letter containing amiable birthday greetings he received last week from the North Korean leader.

Trump also reaffirmed that he has a good relationship with Kim and that North Korea has a "phenomenal future."

White House Press Secretary Sarah Sanders said in a statement on Sunday that correspondence between the two leaders has been ongoing.