(CNN) The release of three US nationals currently detained in North Korea is "imminent," according to an official with knowledge of the ongoing negotiations.

The official told CNN the North Koreans made the decision to free the Americans two months ago, and that North Korea's Foreign Minister Ri Yong Ho had proposed their release during his visit to Sweden in March

US officials insisted at the time that their release "must not be related or used to loosen the main issue of denuclearization," the source said.

The three Americans, Kim Dong Chul, Kim Hak-song and Kim Sang Duk, also known as Tony Kim, have been detained in North Korea for months.

Donald Trump, who looks set to become the first sitting US President to meet a North Korean leader during a planned summit with Kim Jong Un, said last month his administration was fighting "very diligently to get the three Americans back."