Nikki Haley said that President Donald Trump’s “fire and fury” comments last month were not an empty threat.
She said the UN Security Council has run out of options following North Korea’s recent nuclear tests and missile launches.
“We have pretty much exhausted all the things that we can do at the security council at this point,” she told CNN.
“We’re trying every other possibility that we have but there’s a whole lot of military options on the table,” she said.
“None of us want war. But we also have to look at the fact that you are dealing with someone who is being reckless , irresponsible, and is continuing to give threats not only to the United States, but to all their allies, so something is going to have to be done.”
On Sunday, Trump taunted Kim Jong-un — referring to him as “Rocket Man” — by saying the country’s citizens were queuing for petrol because of sanctions.
He tweeted: “I spoke with President Moon [Jae-in] of South Korea last night. Asked him how Rocket Man is doing. Long gas lines forming in North Korea . Too bad!”
The UN security council voted to reduce petrol exports and restrict crude oil supplies to North Korea in response to its sixth nuclear test, the most powerful so far.
Haley said sanctions had caused North Korea to be “cut off from the world”, but there is little evidence of queues for petrol in the country as Trump suggests.
Secretary of state Rex Tillerson said he was waiting the Kim Jong-un to engage in “constructive, productive talks”.
“All they need to do to let us know they’re ready to talk is to just stop these tests, stop these provocative actions, and let’s lower the threat level and the rhetoric,” he said.