Affirming their support for Trump's scheduled June 12 meeting with Kim Jong Un in Singapore, seven influential Democrats including Senator Minority Leader Chuck Schumer warned Trump against signing a deal at any cost, and laid out conditions they want met before any agreement is struck.

"As we approach what could be a historic summit I'm concerned that the president lacks a real strategy in place on North Korea and risks squandering a potentially historic opportunity, putting our security and that of our allies at risk," Senator Robert Menendez, the top Democrat on the Senate Foreign Relations Committee, told reporters.

Schumber added: "We're all hoping the president will succeed in rooting for peace. We very much hope that he'll be able to achieve a lasting and strong agreement.

"But the president needs to be willing to walk away from the table if there isn't a good deal to be had, and he also needs to be willing to take the time to construct a good deal."

In a letter to Trump, the lawmakers wrote that any pact explicitly or implicitly giving Pyongyang sanctions relief without verification of its obligations to dismantle its nuclear and missile arsenal "is a bad deal."

Topping their demands is the elimination of "all nuclear, chemical and biological weapons from North Korea."

Democrats also called on the White House to engage Congress throughout the diplomatic process, and stressed that were the Trump administration to start "veering off course" and sign an unsatisfactory deal, Republican and Democratic lawmakers would act.

"The president has a lot of flexibility when it comes to sanctions but it is always possible for Congress to pass mandatory sanctions or to pass laws to prevent the president from being able to use his waiver authority," Schumer said.

The White House said its advance team featuring military and security staff was already on the ground in Singapore "finalizing preparations" for the summit.