On his way to Washington on Saturday (Feb 10), US Vice President Pence told reporters that "there is no daylight" between the United States, South Korea and Japan in their efforts to isolate North Korea economically and diplomatically until they abandon their nuclear ballistic missile program.
The comments represent a departure from the Trump administration's previous stance that North Korea must first agree to discuss giving up its nuclear weapons before talks began.
"The North Korean leadership simply does not want to give up the nuclear capability it has managed to create and South Korea's engagement policy can hardly persuade them to change their mind", said Balazs Szalontai, an associate professor at Korea University.
The delegation, led by North Korean ceremonial head of state Kim Yong-nam, includes Kim Yo-jong, the younger sister of Kim Jong-un.
The senior diplomatic source said Pence "degraded the image of the United States as a superpower" by meeting with North Korean defectors along with Otto Warmbier's father, and by speaking strongly against North Korea on multiple occasions.
They have shared kimchi and soju with Moon, sat in the same box at the Olympics opening ceremony and cheered a unified women's ice hockey team together.
The invitation came during talks and a lunch Moon hosted with Kim Yo Jong, the younger sister of the North Korean leader, at the presidential Blue House in Seoul.
The rapprochement pushed by the South Korean President has angered some who accuse him of being a North Korea sympathiser.
But the current situation is considerably different from the past, as North Korea has advanced its nuclear and missile programs at an alarming pace.
The statement appears to represent a significant shift in the US position on North Korea - which has until now been that Pyongyang would have to agree to talking about complete, verifiable and irreversible denuclearization.
"I think it is different from the last 20 years", Pence said, according to the Post.
North Korea has made it clear however its nukes are up for negotiation in talks - pledging to continue building nuclear ICBMs.
Another Seoul-based expert said the decision to send a special envoy before holding the summit would help set up a table for dialogue between the USA and North Korea, which is the main agenda of the Moon administration.