U.S. President Donald Trump said Monday he believes his summit with North Korean leader Kim Jong Un will "work out very nicely" as senior officials made last-minute preparations a day before the two leaders sit down together.

"We've got a very interesting meeting in particular tomorrow, and I just think it's going to work out very nicely," Trump told Singapore's Prime Minister Lee Hsien Loong during their talks, according to a pool report.

Trump thanked the prime minister for Singapore's efforts to host the summit, the first-ever between a sitting U.S. president and a North Korean leader, at a hotel on the city-state's resort island of Sentosa on Tuesday.

While the world focuses on whether Trump, who arrived in Singapore on Sunday, will find ways with Kim for North Korea to give up its nuclear weapons, senior officials of the two countries continued working-level negotiations in the final hours.

U.S. Secretary of State Mike Pompeo said on Twitter that those officials held "substantive and detailed meetings" in Singapore, but did not elaborate.

(Resort island of Sentosa)

The U.S. delegation led by Sung Kim, a seasoned diplomat who previously served as ambassador to South Korea and special envoy for North Korea policy, met with senior North Korean officials, including Choe Son Hui, a vice foreign minister long handling Pyongyang's negotiations with Washington.

Before arriving in Singapore, the U.S. and North Korean delegations held a series of substantive discussions in the Demilitarized Zone on the Korean Peninsula.

The two sides are believed to still be at odds over how to realize the goal of complete denuclearization of North Korea.

The United States has sought a "complete, verifiable and irreversible denuclearization," but North Korea has long opposed any unilateral disarmament and suggested that trustworthy security assurances and economic aid should be given first.

The North Korean leader told Lee on Sunday after his arrival in Singapore, "The entire world is watching the historic summit" between him and Trump.

Kim spent Monday out of public view, apparently staying all the time at the heavily guarded St. Regis Hotel.