For months, Washington has been bracing for North Korea to test an intercontinental ballistic missile, waiting to see how close it could get to the United States and how aggressively President Trump should react.

Now, it looks as if Kim Jong-un, the North’s 33-year-old leader, has a different plan — one intended to improve his ability to strike the United States without setting off an American military response.

Instead of going for distance, he has stepped up the testing of missiles that fly high into space — on Sunday, one reached a height of more than 1,300 miles — and then plunge down through the atmosphere, mimicking the kind of fiery re-entries a nuclear warhead would undergo if fired over a much longer distance. Instead, the payload lands in waters a few hundred miles or so from North Korea’s coast.

“They can simulate an ICBM warhead on this kind of trajectory,” David C. Wright, a senior scientist at the Union of Concerned Scientists, a private group in Cambridge, Mass., said in an interview. “It’s a kind of steppingstone.”