South Korea's National Security Adviser Chung Eui-young said Friday that President Donald Trump has sent a congratulatory message to North Korean leader Kim Jong-un on his birthday.

Chung, who met Trump in Washington this week, told reporters that the US president gave him a message to send to North Korea, adding that he had delivered it on Thursday.

"The day we met, it was Kim Jong-un's birthday and President Trump remembered it and asked me to deliver the message," he said after returning to South Korea.

It is believed that Kim's birthday is January 8, although his secretive regime has never confirmed this date. The North Korean leader is 36, according to US government records.

Chung did not say whether the letter was in writing or if it contained anything other than a Christmas greeting.