President Donald Trump has connected military options to what we're trying to politically, he said.

"For too long, those two things were disconnected from each other. So, you need the viable option, the military option, to help make what you were doing diplomatically, economically, with sanctions, viable, to be able to resolve this problem short of what would be, as the president said, a major, major war and a humanitarian catastrophe," McMaster said.

Asked whether the US is willing to take the risk of North Korean retaliatory attacks against Seoul in the event of a US military strike, McMaster said that what Trump cares most about is the safety of the American people.

"What the president has first and foremost on his mind is to protect the American people. And I don't think anyone thinks that it would be acceptable to have this kind of regime with nuclear weapons that can target, that can range the United States," he said.

The official said China's role is important, but other nations should help as well.

"We are going to ask China to do more as we do more, as our South Korean and Japanese allies. But really, all nations have to take a look at this regime. It's already isolated itself, but to further isolate it financially and then also, diplomatically, to make clear that none of us can accept a North Korea with a nuclear weapon," he said. (Yonhap)