North Korean leader Kim Jong Un may have acquired more nuclear weapons since he first met with President Donald Trump in Singapore in June 2018, the Wall Street Journal reported Saturday.

Intelligence analysts have used satellite imagery to detect where nuclear material and instruments are being moved in the country to get an idea of North Korea's nuclear arsenal.

Stanford nuclear scientist Siegfried Hecker said that North Korea could possess between 20 to 60 nuclear bombs. Hecker estimated that North Korea could develop six-to-seven nuclear bombs a year. 

Trump has played down efforts by North Korea to acquire nuclear weapons and also dismissed recent North Korean short-range missile tests as a threat. 

Some Democrats, such as presidential candidate Rep. Tim Ryan, have called Trump's North Korea policy "appeasement."

The U.S. has pursued negotiations over the last year with hopes for Pyongyang to end its nuclear program in exchange for reduced sanctions. Trump and Kim met in Singapore in June 2018 and in Hanoi, Vietnam, in February for negotiations, but no concrete agreement has not been reached.

The two leaders also met at the demilitarized zone between North and South Korea in June, with Trump even stepping into North Korea to meet Kim, making him the first U.S. president to have done so.  

The two have also exchanged friendly letters with one another, with Trump saying he received a "beautiful" letter from Kim for his birthday in June. Kim also received a letter from Trump, which he called "excellent."

The Trump-Kim relationship hasn't always been so friendly, with Trump calling Kim "little rocket man" in front of the United Nations in September 2017 and Kim calling Trump "mentally deranged."