Kim's top diplomat, Ri Yong-ho, told reporters in New York such action could involve a powerful test of a hydrogen bomb in the Pacific.

"We will continue our efforts in the diplomatic arena, but all of our military options -- as the president has said -- is on the table," Tillerson said in an interview on ABC. "And once we can assess the nature of this threat, the president will make a decision regarding the appropriate actions."

Pressed to answer if there could be military action, he said:

"That will be the president's decision. There will be assembled and there is assembled on a standing basis a National Security Council that meets on each of these issues to advise the president. Ultimately, it will be his decision."

Tensions have escalated in the wake of North Korea's repeated missile launches and sixth nuclear test on Sept. 3. Many analysts believe it is only a matter of time before Pyongyang achieves its goal of having a nuclear-tipped missile capable of hitting the continental US. 

A day earlier, Trump announced new sanctions on North Korea that aim to cut off sources of revenue for the regime's nuclear and missile programs.

Tillerson said the measures are the "strongest economic sanctions ever" to target Kim.

"He is being tested with these sanctions," the secretary said.

"Voices from every corner of the world are calling on him to cease his program, come to the table and let's talk about the future of North Korea and the North Korean people."

So far, Pyongyang has not demonstrated an interest in serious discussions, Susan Thornton, acting assistant secretary of state for East Asian and Pacific affairs, said later in a meeting with reporters in New York.

"A lot of our sort of attempts to open up some diplomatic space have been met by additional missile launches with longer ranges or more dramatic nuclear tests," she said. "So I think I wouldn't want to indicate that there's anything other than that kind of a situation on the diplomatic front at the moment."

Should North Korea detonate an H-bomb over the Pacific, she added, it would be an "unprecedented act of aggression."

"I certainly hope that they would not engage in that behavior, but I'm sure there will be a very concerted and determined international response to such an act." (Yonhap)