Seoul, South Korea — South Korea said Friday it conveyed happy birthday wishes from President Trump to North Korean leader Kim Jong Un. His birthday is believed to be Jan. 8.

Returning from a visit to Washington on Friday, Chung Eui-yong, South Korea's presidential national security director, told reporters Mr. Trump asked Seoul to deliver the message to Kim during a meeting at the White House this week.

The Reuters news agency quotes Chung as saying at the airport that, "The day we met was Kim Jong Un's birthday and President Trump remembered this and asked me to deliver the message."

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Chung didn't disclose what the message said, but said Seoul sent it to Pyongyang on Thursday through "proper means."

Kim last week opened the new year expressing deep frustrations over stalled nuclear negotiations with the Trump administration and vowed to bolster his nuclear arsenal as a deterrent against "gangster-like" U.S. sanctions and pressure.

The North in past months has severed virtually all cooperation with the South while demanding that Seoul break away from Washington and restart inter-Korean economic projects held back by U.S.-led sanctions. But the Koreas still operate a liaison office in the North Korean border town of Kaesong.

Seoul had lobbied hard for the resumption of nuclear negotiations, with Chung shuttling between Pyongyang and Washington to help set up the first summit between Kim and Mr. Trump in June 2018.

But negotiations have faltered since the collapse of the second Kim-Trump meeting in February last year, when the U.S. side rejected North Korean demands for major sanctions relief in exchange for a partial surrender of its nuclear capabilities.

Mr. Trump and Kim met again in June and agreed to resume negotiations. But an October working-level meeting in Sweden broke down over what the North Koreans described as the Americans' "old stance and attitude."

Despite the standstill in talks, Mr. Trump and Kim have both described their personal relationship as good. The president has boasted about the "beautiful" letters he has received from Kim.

Pyongyang has never officially confirmed Kim's birth date. While covering a 2014 visit to the country by Dennis Rodman, North Korean state media said the former NBA star organized an exhibition game on Jan. 8 to celebrate Kim's birthday.