President Donald Trump used Twitter on Saturday to once again downplay the possibility of a diplomatic solution to the escalating game of chess with North Korean dictator Kim Jong Un.

“Presidents and their administrations have been talking to North Korea for 25 years, agreements made and massive amounts of money paid hasn’t worked, agreements violated before the ink was dry, makings fools of U.S. negotiators. Sorry, but only one thing will work!” Trump tweeted. He did not say what that “one thing” is.

The United States and North Korea have been in a standoff over Pyongyang’s development of nuclear weapons and ballistic missiles. North Korea has marched closer to achieving a nuclear-tipped missile that could strike America.

Kim Jong Un and Trump have been exchanging personal insults (Trump has called him “Rocket Man” — including in his speech to the United Nations General Assembly) and threats of war with no sign of a diplomatic solution.

Trump has been pressing for tougher measures on Pyongyang from China, the North’s chief trading partner and source of aid and diplomatic support. Although adamantly opposed to steps that could bring down Kim’s regime, Beijing appears increasingly willing to tighten the screws on Pyongyang, and agreed to tough new United Nations sanctions that would substantially cut foreign revenue for the isolated North.

A top North Korean official last month said one of Trump’s tweets amounted to a “declaration of war” and North Korea had the right to retaliate by shooting down U.S. bombers, even in international airspace.

Contributing: Associated Press