A year after US President Donald Trump and DPRK top leader Kim Jong-un met for the first time, the pair appears committed to a closer personal relation, despite a stalemate in efforts to get an agreement on denuclearizing the Korean Peninsula.

On Tuesday, Trump said he had received a "beautiful" and "very warm" letter from Kim, the first known engagement of its kind since the two leaders' last meeting in Vietnam.

"I just received a beautiful letter from Kim Jong-un," Trump told reporters outside the White House. "It was a very personal, very warm, very nice letter. I appreciate it."

Without disclosing the contents of the letter, Trump said that he thinks the Democratic People's Republic of Korea has "tremendous potential" under Kim's leadership.

There have been no reports on the letter from the DPRK's media so far.

Trump and Kim have exchanged multiple letters since the two sides began to engage diplomatically over nuclear issues early last year. Most stress the two leaders' commitment to working toward complete denuclearization, but lack mention of specific steps.

Wednesday marked one year since the two leaders held a historic first summit in Singapore. The second summit between the two, however, collapsed due to differences over the scope of the DPRK's denuclearization and sanctions relief from the United States.

Since then, Pyongyang has complained of US sanctions and Kim said he would wait until the end of the year before deciding on whether to take a "new path".

In May, the DPRK launched short-range missiles, a move that Cheng Xiaohe, an associate professor at Renmin University of China's School of International Studies, said is an apparent "protest over the impasse of denuclearization" .

Trump, however, played down the launches as a possible attempt to "get attention". Trump said he characterized the missile launches as "very short term, short range" and claimed that Kim has kept his promise not to test nuclear weapons or long-range missiles.

"By 'selectively ignoring' Pyongyang's short-range missile launches, Trump's remarks actually show that Washington does not yet have a firm plan for the road map and timetable for denuclearization," Cheng said.

US National Security Adviser John Bolton said that the missile tests violated UN resolutions.

Speaking hours earlier than Trump, Bolton told reporters that the DPRK is not complying with the terms agreed upon at the Singapore summit.

"The Washington administration itself needs to have a consistent position first," Li Chengri, a researcher at the Chinese Academy of Social Sciences said. He thinks that differences within the White House over the DPRK will only slow down the denuclearization process.