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.

Trump has touted his excellent relationship with Kim — even going so far as to say the two had “fallen in love.” But the senior official, a former chief negotiator for the now-defunct six-party talks, said the North Korean leader’s “good personal feelings” for Trump “are, in the true sense of the word, ‘personal.'”

“The Chairman of the State Affairs Commission would not discuss the state affairs on the basis of such personal feelings, as he represents our state and its interests,” he said, referring to Kim Jong Un.

“What is clear is that we will never lose our time again, being taken in by the U.S. trick as in the past,” he said, adding that restarting the talks “may be possible” only under the condition of Washington’s “absolute agreement” on the issues raised by the North in previous talks.

Still, he remained skeptical of the U.S. accepting such demands, saying: “We know well that the U.S. is neither ready nor able to do so. We know… the way we should go and will go on our way.”

The U.S. president and North Korean leader have held three meetings since June 2018 but negotiations have been largely deadlocked since their Hanoi summit last February collapsed due to major differences over the scope of the North’s denuclearization and potential sanctions relief by the United States. In Hanoi, the North had reportedly offered to dismantle its main Yongbyon nuclear facility in exchange for the elimination of key U.N. sanctions over its nuclear weapons program.

But, in what could be a shift to a more hardline stance, Kim Kye Gwan used Saturday’s statement to clearly note Pyongyang’s evolving position in terms of what it is willing to offer up in the high-stakes negotiations.

“There will never be such negotiations as that in Vietnam, in which we proposed exchanging a core nuclear facility of the country for the lift of some U.N. sanctions in a bid to lessen the sufferings of the peaceable people even a bit,” he said.

The latest statement comes just weeks after Kim Jong Un expressed deep frustrations in a recent key policy speech over the 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.

He also declared that his country is no longer bound by its self-imposed moratorium on nuclear and intercontinental ballistic missile tests, while also warning of a “new strategic weapon” that he vowed to soon reveal to the world.

The self-imposed halt to such tests had been a centerpiece of the unprecedented nuclear diplomacy between Pyongyang and Washington over the past two years, which saw little tangible progress.

Saturday’s statement came a day after South Korea’s national security adviser said Trump had asked Seoul to deliver his birthday message to Kim Jong Un.

Kim’s birthday is believed to be Jan. 8, though his secretive regime has never confirmed the date. The U.S. government lists Kim’s birth year as 1984, making him 36 years old this year.

The North Korean adviser also used his statement to caution South Korea to steer clear of ties between the North and the United States, saying it should not seek “to play a mediator role.”

For South Korea to meddle in personal relations between Kim and Trump was “presumptuous,” the North Korean official said, adding that the North had already directly received from Trump a letter with the greetings.

“But they seem not to know that there is a special liaison channel between the top leaders of the DPRK and the United States.” It was not clear what “channel” this was referring to, but Kim and Trump have been known to exchange letters.

DPRK refers to the official name of North Korea, the Democratic People’s Republic of Korea.