US President Donald Trump says he had "a long and very good talk" with South Korean President Moon Jae In, one day after Moon and his North Korean counterpart signed an agreement to pursue a nuclear-free peninsula and an official end to the Korean War.
"Things are going very well," Trump said in a tweet, adding that a time and location for his own meeting with North Korean leader Kim Jong Un "is being set".
The White House confirmed the phone call.
"President Donald J Trump spoke today with President Moon Jae in of the Republic of Korea to discuss the April 27 meeting between President Moon and North Korean leader Kim Jong Un," according to a White House statement.
The two leaders agreed that "unprecedented pressure" applied by the US, South Korea and the international community had led to what it termed "this significant moment".
Trump and Moon emphasised that "a peaceful and prosperous future for North Korea is contingent upon its complete, verifiable, and irreversible denuclearisation," the statement added.
Earlier, the the news agency Yonhap quoted a statement from the South Korean president's office indicating the call lasted one hour and 15 minutes.
Trump said he also spoke by phone with Japanese Prime Minister Shinzo Abe "to inform him of the ongoing negotiations," and the Pentagon said US Secretary of Defence James Mattis spoke with South Korean Defence Minister Song Young-moo.
The White House said Trump and Abe affirmed their commitment to achieving "the permanent and verifiable denuclearisation of North Korea" and to continuing their close coordination in advance of talks between the United States and North Korea.
They also reiterated the need for North Korea to abandon all weapons of mass destruction and ballistic missile programs, according to a White House statement.
North and South Korea agreed at the summit to pursue trilateral talks with the US, or quadrilateral meetings with the US and China, to declare an end to the Korean War, turn the armistice into a peace treaty and establish "a permanent and solid peace regime".
The thaw this year follows a tense 2017, in which multiple missile tests by North Korea prompted international outcry and an exchange of insults between Trump and Kim.
Trump now credits his tough talk and the relationship he has built with Chinese President Xi Jinping for bringing about changes in Kim's posture. The US president also says that "maximum pressure," including sanctions and other economic isolation, will continue until North Korea completes denuclearisation.