Moon and Trump are set to meet on Tuesday in Washington before North Korean leader Kim Jong Un meets with Trump on June 12 in Singapore.
Although a historic inter-Korean summit in late April raised hopes of reconciliation, North Korea showed a dramatic change in tone in recent days.
North Korea's chief negotiator Ri Son Gwon said on Thursday it would not hold talks with South Korea unless their demands were met, taking issue with the U.S.-South Korean air combat drills known as Max Thunder. It came a day after it threatened to pull out of the summit with the United States.
Further dampening the mood, a spokesman for North Korea's Red Cross Society demanded on Saturday that South Korea's government should send North Korean female restaurant workers back to their home "without delay" to show the will to improve the inter-Korean ties, the North's Korea Central News agency said.
A dozen North Korean restaurant workers came to South Korea in 2016 from China, and North Korea had urged to send them back claiming they were abducted by the South, even though the South has said the 12 workers decided to defect of their own free will.
Lee Dong-bok, a researcher at New Asia Research Institution, said part of the reason for the North's demands of the repatriation is to divide South Korea's public opinion over the 12 workers.
"It is also to pressure the Moon government to agree to its demand so that South Korea can keep up the momentum for the North Korea-U.S. summit meeting," Lee said.