The comments followed an emergency Security Council meeting on the escalating Korea crisis.
The North Korean missile tests have angered South Korea and Japan and prompted the United States to take swift action.
US Ambassador to the UN, Nikki Haley, denounced North Korean leader, Kim Jong-un, after a Security Council meeting in New York.
"We have to see some sort of positive action taken by North Korea before we can ever take them seriously. They've given us enough reason to think how irresponsible that they are, that we would ever try and think that we are dealing with a rational person on this."
US State Department spokesman Mark Toner says the onus is on Pyongyang.
"We remain open to dialogue with North Korea with the aim of returning to credible and authentic negotiations on the denuclearisation of the Korean peninsula. But, and I'll be very clear about this: the onus is on North Korea to take meaningful actions toward denuclearisation and refrain from provocations."
China has called on North Korea to suspend its missile tests, saying in return, South Korea and the US should halt joint military exercises.
China says the drills across the border in South Korea were increasing tensions and the use of the US system's powerful radars were seen as a security threat.
The US rejected China's proposal.
US military forces have already begun moving equipment for its controversial missile defence system known as THAAD to ally South Korea.
Nikki Haley says it was a move to protect South Korea, not one directed towards China.
"In terms of the THAAD with South Korea, tell me why we wouldn't do the THAAD in light of 24 ballistic missiles, in light of two nuclear tests, knowing that we're going to protect our allies. We are not going to leave South Korea standing there, with the threat of North Korea facing them and not help."
International pressure on North Korea has increased recently, with the assassination of Kim Jong Un's half-brother at Kuala Lumpur International Airport last month.
South Korean intelligence officers say the North Korean leader had issued standing orders for the elimination of his elder half-brother.
A video has emerged of a man claiming to be Kim Jong Nam's son.
The 21-year-old said he is in hiding with family members.
"Hi. My name is Kim Han Sol from North Korea, from the Kim family. Here is my passport. My father has been killed few days ago. I am currently with my mother and sister."
