The concern is NOT that the United States will start by attacking North Korea with nuclear weapons. The concern is that any attack by the United States, no matter how conventional, will prompt a nuclear response from North Korea.
The truth is that Seoul, a sprawling city of 26 million people in its is so close to North Korea that it would be challenging for American technology to stop a nuclear attack on it. We might, but we also might not. The same is true for Japan. There are not any guarantees for this except that NK would almost certainly try.
Even if kept non-nuclear Seoul is in easy artillery range of North Korea and they have an unbelievable crap ton of artillery.