The US president also says China’s Xi Jinping is “trying very hard” to end the Korean crisis.
US President Donald Trump has said he would like to solve the North Korea crisis diplomatically, but that a “major, major conflict” is possible.
In an interview with Reuters he praised China’s President Xi Jinping for his handling of North Korea, calling him “a very good man” who loved his country.
Mr Trump also said it had been “very hard” for Kim Jong-un to take over North Korea at such a young age.
The UN Security Council is meeting to discuss North Korea on Friday.
US Secretary of State Rex Tillerson said Washington would negotiate with North Korea, with a view to removing nuclear weapons from the country, not changing the government.
He told NPR radio in the US: “We do not seek regime change, we do not seek a collapse of the regime, we do not seek an accelerated reunification of the peninsula.
“We seek a denuclearised Korean peninsula – and that is entirely consistent with the objectives of others in the region as well.”
Mr Tillerson also indicated that he thought China, North Korea’s major ally, might be starting to see the regime as a “liability” or a security risk.
“What China is beginning to re-evaluate is whether North Korea is any kind of an asset to them, or whether North Korea themselves and the regime have become a liability to China’s own security, ” he said.
Shortly after being elected, Mr Trump accused China of not doing enough to rein in North Korea and suggested the US could take unilateral action.
But Chinese Foreign Minister Wang Yi has warned that tensions on the Korean peninsula risk getting out of control.
According to Mr Tillerson, China has told the US it will impose sanctions on North Korea if it conducts further nuclear tests.
In his wide-ranging interview inside the Oval Office, Mr Trump – who met Mr Xi earlier this month – said the Chinese president “certainly doesn’t want to see turmoil and death”.
“He is a very good man and I got to know him very well, ” he said.
“He loves China and he loves the people of China. I know he would like to be able to do something, perhaps it’s possible that he can’t.”
Of Mr Kim, he said: “He’s 27 years old. His father dies, took over a regime. So say what you want but that is not easy, especially at that age.”
But he stressed he was “not giving him credit”, and added: “I hope he’s rational.”
“There is a chance that we could end up having a major, major conflict with North Korea. Absolutely, ” said Mr Trump. North Korea has carried out repeated missile tests in recent months and is threatening to conduct its sixth nuclear test.
Other developments have raised tensions in recent weeks:
Earlier on Thursday, Mr Tillerson told Fox News that China had again urged North Korea to refrain from carrying out more tests.
In February, China banned coal imports from North Korea – one of the country’s key exports – and is reportedly also considering restricting oil shipments if Pyongyang continues to behave belligerently.
The president reflected on how the demands of his job were greater than his “previous life”, which he said he had loved.
“You’re really into your own little cocoon, because you have such massive protection that you really can’t go anywhere, ” he said.
“I like to drive. I can’t drive any more.”
On other key topics, Mr Trump said:
Earlier, Russia’s President Vladimir Putin called for the resumption of talks with North Korea.
Speaking in Moscow, where he met Japanese Prime Minister Shinzo Abe, he urged those involved to “refrain from using belligerent rhetoric”.
