SEOUL — President Trump sent a letter to Kim Jong Un to offer help in North Korea's battle against the coronavirus, Kim's sister said Sunday, but she warned it wasn't enough to improve relations.

The United States and North Korea are at loggerheads over the reclusive state's nuclear weapons program. Trump has attempted to restart negotiations by cultivating a personal relationship with Kim.

“We view such a personal letter of President Trump as a good example showing the special and firm personal relations with Chairman Kim Jong Un,” said Kim Yo Jong, the leader’s sister and a senior official of the ruling party. Her statement was published by North Korea’s official Korean Central News Agency.

Kim Yo Jong said Trump explained in the letter “his plan to propel the relations between the two countries of the DPRK and the U.S.,” a reference to the country’s formal name of the Democratic People’s Republic of Korea.

The U.S. president also explained “his intent to render cooperation in the anti-epidemic work,” the state news agency reported. It quoted Trump as saying he was “impressed by the efforts made by the chairman to defend his people from the serious threat of the epidemic.”

Trump’s letter was “consistent with his efforts to engage global leaders during the ongoing pandemic,” said a senior Trump administration official, who spoke on the condition of anonymity to discuss private diplomatic discussions. “The President looks forward to continued communications with Chairman Kim.”

Coronavirus emerged in neighboring China in January, but North Korea has not reported a single case. The isolated country has adopted stringent quarantine measures against the virus, calling it a matter of “national survival.”

Kim’s sister warned against drawing “hasty conclusions” from the good personal relationship between the two leaders. “I think that the bilateral relations and dialogue for them would be thinkable only when the equilibrium is kept dynamically and morally and justice is ensured between the two countries, not merely by the personal letter between the two leaders,” Kim Yo Jong said in her statement.

Duyeon Kim, a senior adviser at the International Crisis Group, said the statement was North Korea’s way of saying it still wants a better offer at the negotiating table.

“North Korea is warning that they won’t be fooled by niceties or U.S. tactics and that the U.S. needs to do better,” she said. She said the North Korean leader’s sister was “making it clear that Pyongyang will continue with nuclear weapons development.”

This is the second known correspondence this year between Trump and Kim, after Trump sent birthday greetings in January.

Kim Kye Gwan, an adviser to North Korea’s Foreign Ministry, downplayed the diplomatic significance of that letter at the time, saying such personal exchanges between the leaders were not enough to draw Pyongyang back into nuclear talks.

Negotiations between the two countries over the denuclearization of North Korea have been stalled since the collapse of a summit in February 2019. North Korea demanded sanctions relief in exchange for partial disarmament steps.

Trump had a brief encounter with Kim at a border town between the South and North in June. The leaders at the time touted “a very good relationship,” but it failed to produce progress in disarmament. North Korea resumed weapons tests within a month.

On the day the Korean Central News Agency reported on Trump’s letter, the state media outlet also said Kim had guided a successful test of “tactical guided missiles” the previous day.

“The timing almost certainly was carefully calibrated, possibly to underscore North Korea’s message that it remains skeptical about the prospects of improved U.S.-DPRK ties,” analyst Rachel Minyoung Lee wrote on the NK Pro website.

John Hudson contributed to this report.