The leader of the most isolated nation in the world has invited President Donald Trump to meet, and the American commander-in-chief has accepted the invitation—a potential breakthrough in tensions that have threatened to escalate into a war between two nations equipped with nuclear weapons..

In a letter penned by North Korean leader Kim Jong Un and delivered to Trump by a South Korean delegation that visited the White House on Thursday, Kim vowed to refrain from continuing missile tests that had prompted an escalation in U.S. sanctions against the country, and expressed “eagerness to meet President Trump as soon as possible,” in the words of The delegation leader Chung Eui-yong.

Trump, in turn, said that he would meet Kim “by May,” according to Chung, who declared that South Korea and its partners were “optimistic” about diplomatic process succeeding in the future—so long as the government in Pyongyang “matches words with concrete actions.”

Chung also lauded the muscular sanctions lodged against the North Korean regime by the United States in recent months as having “brought us to this juncture.” Those sanctions targeted ships, companies with what one person in what the Trump administration called “the largest package of new sanctions against the North Korea regime” in the hopes of persuading Pyongyang to end its nuclear weapons program.

Following Chung’s statement, White House press secretary Sarah Huckabee Sanders released a statement, noting that “President Trump greatly appreciates the nice words of the South Korean delegation and President Moon,” and that Trump “will accept the invitation to meet with Kim Jong Un at a place and time to be determined.”

In the meantime, Sanders noted, “all sanctions and maximum pressure must remain.”

The president had earlier told reporters in the White House briefing room that South Korea planned to make “a major announcement” on Thursday evening.

Trump had tweeted on Tuesday morning that “possible progress” was being made in talks about the dictatorship’s nuclear arsenal. “For the first time in many years, a serious effort is being made by all parties concerned,” Trump wrote. “The World is watching and waiting! May be false hope, but the U.S. is ready to go hard in either direction!”

Trump’s willingness to meet directly with Kim may spark cautious optimism among some who worried that the president’s more bellicose comments previewed a potential conflict. In October, Trump dismissed Secretary of State Rex Tillerson as “wasting his time” in attempting to engage in direct talks with North Korean leadership. Trump had also declared in August that any overt aggression by Pyongyang would be “met with fire and fury like the world has never seen.”

“North Korea best not make any more threats to the United States,” Trump said at the time. Kim, he continued, “has been very threatening... and as I said they will be met with fire, fury and frankly power, the likes of which this world has never seen before.”

In January, the CIA briefed Trump on the feasibility of a limited attack on North Korea.

“We’re working to prepare a series of options to make sure that we can deliver a range of things so the president will have the full suite of possibilities,” CIA Director Mike Pompeo told the conservative American Enterprise Institute’s Marc Thiessen about his conversations with the president.

“We are equally at the same time ensuring that if we conclude that is not possible, that we present the president a range of options to achieve what is his stated intention.”