US President Donald Trump has sent a , South Korea's national security adviser Chung Eui-Yong said.
Mr Chung, who met Mr Trump in Washington this week, told reporters that he was given a message to pass to North Korea and it was delivered on Thursday.
The advisor said on arriving back in South Korea that the US President had remembered the day they met was the North Korean leader's birthday and had sent him birthday greetings.
"President Trump asked President Moon to deliver his message to Chairman Kim," said Mr Chung, without elaborating whether it was a written message or whether it included anything beyond the greeting.
Mr Kim's birthday is believed to be 8 January, though his secretive regime has never confirmed the date. The US government lists the leader's birth year as 1984, making him 36 years old this year.
On Wednesday, Mr Chung also met the US special envoy for North Korea, Stephen Biegun, and "reaffirmed close US-ROK coordination on North Korea," State Department spokeswoman Morgan Ortagus said, using the initials for South Korea's official name the Republic of Korea.
The two also discussed recent events in the Middle East and their coordination on global security issues.
South Korean Foreign Minister Kang Kyung-wha will meet US Secretary of State Mike Pompeo in California next week, along with their Japanese counterpart, Toshimitsu Motegi, and North Korea and South Korea-Japan relations will be on the top of the agenda.
South Korean President Moon Jae-in said on Tuesday there is an urgent need for practical ways to improve ties with North Korea, adding that he was ready to meet its reclusive leader in North Korea.
