The United States is negotiating with North Korea to release a joint document when their leaders meet for a historic summit in Singapore on June 12, U.S. government sources said Wednesday.

The focus of the talks between U.S. President Donald Trump and North Korean leader Kim Jong Un will be to what extent of "complete denuclearization" Kim will agree. But reaching a deal on the issue will not be easy, leaving uncertainty on whether a joint document will be released.

Donald Trump (Getty/Kyodo) Kim Jong Un (KNS/Kyodo)

In remarks apparently trying to lower expectations for the outcome of the summit, Trump said last week that he believes Kim is committed to denuclearization, but, "We're not going to go in and sign something on June 12th, and we never were. We're going to start a process."

The possible issuance of the joint document has apparently been discussed during working-level talks that have taken place between U.S. and North Korean delegates at the border village of the two Koreas to prepare for the Trump-Kim talks.

The sources said the United States is hoping to come up with a document, drawing on a joint statement issued in 2005 at the six-party talks involving the two Koreas, China, the United States, Japan and Russia aimed at curbing the North's nuclear ambitions.

In that statement, North Korea committed to abandoning all nuclear weapons and existing nuclear programs and returning to the Nuclear Non-Proliferation Treaty from which it declared in 2003 it was withdrawing.

It also promised to accept safeguard inspections by the International Atomic Energy Agency.

The United States, for its part, affirmed that it had no nuclear weapons on the Korean Peninsula and had no intention to attack or invade North Korea with nuclear or conventional weapons.

The six-party talks effectively collapsed after the last round in 2008.