The United States and North Korea will focus on a proposed shipment overseas of some of Pyongyang's nuclear warheads during preparatory talks for a summit between President Donald Trump and the North's leader, Kim Jong Un, U.S. officials said Sunday.

The U.S. has demanded that the North ship up to 20 warheads overseas at an early date as part of measures to achieve a "complete, verifiable and irreversible" denuclearization of the country, according to officials. However, reaching a deal is likely to be a challenge given the North's reluctance to meet such a demand.

According to the U.S. officials, who spoke on condition of anonymity, North Korea has expressed a reluctance to ship all of its nuclear weapons and missiles outside the country.