The mistake came in a transcript of a briefing U.S. Secretary of State Mike Pompeo gave on Monday.
There have been plenty of talking points from the historic summit between Donald Trump and Kim Jong-un.
The State Department is declining to release any additional information about the calls.
About 28,500 US troops are now stationed in South Korea as a legacy of the 1950-53 Korean War.
During the signing ceremony, the two leaders sat side by side at an expansive wooden table decorated with white flowers and were flanked on either side by American and North Korean flags, The new Straits Times reported.
"President Trump committed to provide security guarantees to the DPRK and Chairman Kim Jong un reaffirmed his firm and unwavering commitment to complete denuclearisation of the Korean Peninsula".
Trump and Kim met on Tuesday for a historic diplomatic summit.
A large contingent of Democrats criticized the president and his staff for tentatively granting a number of concessions to the North Koreans, including a pause on military exercises in South Korea and even meeting with Kim in the first place to lend his leadership legitimacy on the worldwide stage.
A woman walks past a cannon barrel stuffed with flowers in a symbolic gesture of peace at Fort Siloso in Singapore, as preparations continue for Tuesday's Trump-Kim summit. North Korea is seeking a security guarantee - possibly including a peace treaty formally ending the Korean War - and the removal of the U.S.'s nuclear umbrella protecting allies South Korea and Japan. We fly in bombers from Guam... to practice and drop bombs all over the place and then go back to Guam.
Members of the USA delegation, including U.S. secretary of state Mike Pompeo, right, and John Bolton, national security adviser, left, and North Korean advisers watch a television screen showing the meeting.
In response to a question on whether the two leaders will meet again, Trump said: "We'll meet again, we'll meet many times".
But "we will be stopping the war games. unless and until we see the future negotiation is not going along like it should", he said.