U.S. President Donald Trump reiterated his intention to meet with the Democratic People’s Republic of Korea (DPRK)’s leader Kim Jong Un by the end of May in a phone call with South Korean President Moon Jae-in on Friday.

Trump and Moon discussed the ongoing efforts to prepare for their upcoming engagements with the DPRK, the White House said in a statement.

The two leaders expressed “cautious optimism” over recent developments and emphasized that a brighter future is available for Pyongyang “if it chooses the correct path.”

Both leaders affirmed the importance of “learning from the mistakes of the past,” and pledged to continue close coordination to maintain maximum pressure on the DPRK government.

They also agreed that “concrete actions, not words, will be the key to achieving permanent denuclearization of the Korean Peninsula.”

The DPRK launched several missile and nuclear tests last year, while the United States and South Korea have exerted the “maximum pressure” against the DPRK and held military drills around the Korean Peninsula, further escalating tensions.

Earlier in March, Trump agreed to meet Kim by May at the invitation of the DPRK leader “to achieve permanent denuclearization.”