North Korea pledged this week to halt nuclear and missile tests as the two countries prepare for a summit between Kim Jong Un and Trump, but has not committed to giving up its nuclear weapons -- which Pyongyang views as a shield against the Western overthrow of its government.

"We haven't given up anything & they have agreed to denuclearization (so great for World), site closure, & no more testing!" Trump wrote on Twitter, apparently responding to remarks made by an NBC news host.

"We are a long way from conclusion on North Korea, maybe things will work out, and maybe they won't - only time will tell....But the work I am doing now should have been done a long time ago!"

Kim said Saturday that his country would halt testing and close a nuclear test site, which Trump hailed as "big progress."

But observers warn those concessions could be easily reversed if Kim decides to do so.

Some experts meanwhile view the planned summit itself -- and the legitimacy a meeting with a sitting US president confers on Kim and his government -- as a win for Pyongyang.

"This is a great public relations effort by Kim Jong Un and I think people recognize that. I think everyone within the administration and Congress approaches this with skepticism and caution," Bob Corker, the Republican chair of the Senate Foreign Relations Committee said on CNN's State of the Union.

"You can easily reverse that and all of us know that," said the senator, noting that Kim has begun "the meetings in a way that almost put the United States on the defensive."