"We have indicated to North Korea that we're prepared to sit down and talk without preconditions about their nuclear program," Sullivan told "Face the Nation" on Sunday, in comments coming just days after the north test launched an ICBM.

For months, South Korean and Western officials have expressed alarm over what they see as a potential North Korean nuclear test happening soon. But Sullivan in his own fresh remarks said he's not seen "any immediate indications" of this happening imminently.

"But it would not come as a surprise that North Korea moved forward with another nuclear test," he said.

"With respect to its intercontinental ballistic missile capability, this is a capability they began testing several years ago," Sullivan continued. "They have continued to test it. We watch all of those tests very closely to see how it is developing and we could coordinate extremely closely with our allies — with Japan and Korea —  to make sure that we are responding in lockstep to this threat." 

Sullivan called out China as well, warning of a regional escalation in nuclear build-up activities...

"We've also made clear to China that it is the United States who is ready for diplomacy and North Korea who was not," Sullivan said.