North Korean leader Kim Jong Un reopened a key cross-border communication channel with South Korea for the first time in nearly two years Wednesday as the rivals explored the possibility of sitting down and talking after months of acrimony and fears of war.

The sudden signs of easing animosity, however, came as President Donald Trump threatened Kim with nuclear war in response to his threat earlier this week and a day after U.S. officials said the North could be preparing for another intercontinental ballistic missile test.

In his New Year’s address Monday, Kim said he was willing to send a delegation from North Korea to next month’s Winter Olympics in South Korea. But he also said he has a nuclear button on his desk and that all U.S. territory is within striking distance of his nuclear weapons, comments Mr. Trump latched onto Tuesday when he boasted of a bigger and more powerful “nuclear button” than Kim’s.

The two leaders exchanged crude insults last year, as the North received new U.N. sanctions over its sixth and most powerful nuclear test explosion and a series of intercontinental ballistic missile launches.

The recent softening of contact between the rival Koreas may show a shared interest in improved ties, but there’s no guarantee tensions will ease. There have been repeated attempts in recent years by the rivals to talk, but even when they do meet, the efforts often end in recriminations and stalemate.

Outside critics say Kim may be trying to use better ties with South Korea as a way to weaken the alliance between Washington and Seoul as the North grapples with toughened international sanctions over its nuclear and missile programs.