The United States has rejected China's proposal for a halt to joint US-South Korean military exercises if North Korea suspends its nuclear and missile activities.

It yesterday called North Korean leader Kim Jong Un irrational and demanded "positive action" before the US can take his regime seriously.

In Washington, US State Department acting spokesman Mark Toner said, "At this point we don't see it as a viable deal." 

A Pentagon spokesman, Cmdr Gary Ross, said US activities to defend South Korea "cannot be equated to North Korea's repeated violations of its obligations and agreements." 

The US ambassador to the United Nations, Nikki Haley, told reporters after an emergency Security Council meeting on North Korea's latest ballistic missile launches that the United States must see "some sort of positive action" by Kim's regime before discussing ways to reduce tensions on the Korean Peninsula.

"They've given us enough reason to think how irresponsible that they are that we ever try and think that we're dealing with a rational person on this," she said.

Yesterday, Chinese Foreign Minister Wang Yi proposed the freeze-for-freeze, likening escalating tensions between the North and Washington and Seoul to "two accelerating trains, coming toward each other with neither side willing to give way." 

The idea was rejected by South Korea and Japan as well as the US.

Haley said the military drills are especially needed now after North Korea conducted two nuclear tests and 24 ballistic missile launches last year and two sets of missile launches and the assassination of Kim Jong Un's estranged brother using a chemical weapon this year.

She also defended the upcoming deployment of a US missile defense system in South Korea, a move that has been strongly opposed by China. She said America would not leave its ally facing the threat from North Korea without help.

"We have not seen any goodwill at all coming from North Korea," Haley said. "I appreciate all my counterparts wanting to talk about talks and negotiations, (but) we are not dealing with a rational person." 

With any other country, the United States would be seeking negotiations, she said.

"This is not a rational person, who has not had rational acts, who is not thinking clearly," Haley said of North Korea's leader. "This is someone who is trying to get attention. This is someone who is trying to get a reaction." 

Haley said the United States is re-evaluating how it is going to deal with North Korea going forward "and we are making those decisions now and will act accordingly." 

"We're not ruling anything out and we're considering every option that's on the table," she said.