The US government is not seeking a regime change in North Korea, the secretary of state says, amid tensions over Pyongyang's weapons programme.
"We're not your enemy," Rex Tillerson said, adding that the US wanted a dialogue at some point.
But a Republican senator said President Donald Trump had told him there would be a war with North Korea if its missile programme continued.
Pyongyang claimed its latest missile could hit the US west coast.
The second test of an intercontinental ballistic missile (ICBM) on Friday, celebrated by North Korean leader Kim Jong-un, was the latest to be conducted in defiance of a United Nations ban.
"We do not seek a regime change, we do not seek the collapse of the regime, we do not seek an accelerated reunification of the peninsula, we do not seek an excuse to send our military north of the 38th parallel," said Mr Tillerson, referring to the border between the Koreas.
"We're not your enemy, we're not your threat but you're presenting an unacceptable threat to us and we have to respond."
President Trump has repeatedly criticised China, which shares a land border with North Korea and is its closest economic ally, for not doing enough to stop Pyongyang's weapons programme.
However, Mr Tillerson took a more diplomatic approach, saying that "only the North Koreans are to blame for this situation".
"But," he added, "we do believe China has a special and unique relationship, because of this significant economic activity, to influence the North Korean regime in ways that no one else can."
In a separate development, Republican Senator Lindsey Graham said that Mr Trump told him that there would be a military conflict between the two countries if Pyongyang continued with its aim to develop a missile programme with the US in its range.
"There will be a war with North Korea over their missile programme if they continue to try to hit America with an ICBM.
"He [Trump] has told me that, I believe him, and if I were China I would believe him too, and do something about it," he said in an interview with NBC's Today programme.
Mr Graham added: "If there's going to be a war... it will be over there. If thousands die, they're going to die over there. They're not going to die here. And he [Trump] has told me that to my face."