The commander of the U.S. nuclear arsenal says he would not blindly carry out an ordered strike by President Donald Trump.

General John Hyten says he's well aware of the rules of engagement with nuclear weapons, telling an audience of senior political, military and diplomatic officials gathered in Halifax that he's thought about this a lot.

"If you execute an unlawful order you will go to jail, you could go to jail for the rest of your life," he said during a panel discussion at the Halifax International Security Forum.

"The way the process works is simple. I provide advice to the president and he will tell me what to do. And if it's illegal… I'm going to say 'Mr. President, it's illegal' and you know what he's going to do. He's going to say 'What would be legal?' and we'll come up with options, a mix of capabilities to respond to whatever the situation is," Hyten said.

"And that's the way it works."

Tensions over North Korea's nuclear intentions remain high, and there are concerns that Trump's exchange of threats and personal insults with North Korea's Kim Jong-un makes the situation even more unstable.

Providing room for diplomacy

Hyten says his role is to provide the room for diplomacy and sanctions to work.

"We are ready every minute of every day to respond to any event that comes out of North Korea," he said. "That's the element that has to be clear. And it is clear. If he [Kim] goes down that path, it will not end well."

But those international efforts, especially diplomacy, don't appear to be working, says Sung-han Kim, director of the Ilmin International Relations Institute at Korea University, in Seoul.

He told the summit North Korea has deployed nearly a thousand short-range ballistic missiles and is close to being able to fire a nuclear missile capable of hitting targets in North America.

"For now we are depending on the extended deterrence, the nuclear umbrella provided by the United States," he told the summit. "But it's time to think about plan B, some other alternatives, when we all agree on the fact that our diplomatic efforts have been a failure."

One option is to re-deploy U.S. nuclear weapons on the peninsula. Another is for South Korea to have its own nuclear weapons.

Hyten doesn't agree. He was in South Korea six weeks ago to guarantee the U.S. deterrent is there for them.

Aging nuclear capabilities

"I want to make sure that is understood by our allies because that is the best posture. In my opinion, the more nuclear weapons we have in the world, the less secure the world is."

That said, Hyten warns the Americans' nuclear capabilities are aging, and will need to be replaced in the next decade or two.

"Our Minuteman ICBM eventually, it has to be replaced. Submarines can be built for amazing capabilities, but at some point they don't go under the water anymore because they have gone under so many times," he said.

"We are coming to the end of the lives of a lot of our critical capabilities. And we have to modernize."