The sense of unease in the Japanese government over a thaw between North and South Korea heightened Tuesday with Seoul's announcement that the two Koreas will hold a leaders' summit at the end of next month.

The fear in Tokyo is now that the United States, the shared defense ally of Japan and South Korea, will agree to dialogue with the North without preconditions -- something a Japanese Foreign Ministry source said would leave Japan "out of the loop."

(Photo courtesy of the South Korean presidential office)

Japan has been trying to write the script for the international community to "maximize pressure" on North Korea, insisting that no dialogue should be held with the North until it agrees to abandon its nuclear and ballistic missile programs.

Japanese officials are now preparing to change tack should the situation shift dramatically and compromise the relevance of the pressure campaign, according to government sources.

After talks held in Pyongyang on Monday between North Korean leader Kim Jong Un and envoys representing South Korean President Moon Jae In, the South Korean government said North Korea expressed its readiness to talk with the United States about denuclearization and the normalization of bilateral relations.

Ahead of this announcement, the Japanese government's top spokesman struck a wary tone about the envoys' meeting with Kim.

"When dealing with North Korea, we should reflect fully on the lesson that past dialogue has not led to denuclearization," Chief Cabinet Secretary Yoshihide Suga told a press conference Tuesday morning.

That remark was in keeping with what the administration of Prime Minister Shinzo Abe has been arguing on the international stage for months: that North Korea has a track record of taking advantage of dialogue frameworks to buy time for developments in its nuclear program.

The Japanese government has raised as an example the 1994 "Agreed Framework" between the United States and North Korea, by which the North was supposed to freeze its nuclear program and be provided with alternative energy sources. North Korea continued to enrich uranium despite the framework.

Similarly, North Korea went back on a 2005 commitment to abandon all nuclear weapons, made in a joint statement at the six-party talks between North and South Korea, Japan, the United States, China and Russia. It carried out its first nuclear test the following year.

"This is proof that we were deceived by North Korea," a senior Foreign Ministry official said.

But very few in the Japanese government now believe that South Korea is on the same page as Japan on this matter.

Foreign Minister Taro Kono expressed a hope Monday that the South Korean envoys and Kim could "take a step toward denuclearization" in their talks, but warned the South "not to be dazzled by North Korea's 'smile diplomacy'."

On Tuesday, he suggested Kim's treatment of the envoys suggests the economic sanctions on North Korea are having an effect.

The United States is the true focus of Japan's efforts now, with a State Department spokeswoman hinting last month at the possibility of Washington engaging in "preliminary" talks with the North.

Kono is set to visit the United States later this month, meeting Secretary of State Rex Tillerson in Washington to counsel him to stay committed to the pressure campaign.

"If the United States swallows South Korea's argument that it should place importance on dialogue, Japan will be put in an awkward position with its pressure stance," a Foreign Ministry source said.

At the same time, the Abe administration is aware that maintaining too rigid of a stance could risk Japan becoming isolated if Washington and Pyongyang do start exchanges, resulting in a slight softening of Japan's position.

Abe, despite having long asserted that "pressure, not dialogue, is what is needed," briefly exchanged words with North Korea's ceremonial leader Kim Yong Nam at an event in Pyeongchang, South Korea, while both were there for the opening ceremony of the Winter Olympics last month.

Kono, who called in September last year for all countries to break off diplomatic relations with North Korea, told reporters last month that "making contact with North Korea is important in order to tell it to abandon its nuclear and missile programs and come to the table for dialogue."

According to a Japanese government source involved in North Korea policy, "the situation on the Korean Peninsula is anyone's guess."

"We have to be prepared for everything, because anything could happen," the source said.