Kim Jong Un's personal relationship with U.S. President Donald Trump may not be enough to salvage stalled nuclear talks between Pyongyang and Washington, a senior North Korean official said Saturday.

Kim Kye Gwan, an adviser to the North's Foreign Ministry, said in a statement carried by the state-run Korean Central News Agency that his country had directly received Trump's letter wishing a happy birthday to leader Kim Jong Un, but warned that it will return to nuclear talks only when Washington fully accepts its demands.

"As acknowledged by the world, it is true that the personal relations between (Kim Jong Un) and President Trump are not bad," he said. However, he continued, it would be "absent-minded" to expect Pyongyang to resume dialogue because of that warm personal relationship.