Nikki Haley also says any country that doesn't implement United Nations sanctions is supporting Pyongyang.
Pyongyang says that it needs atomic weapons to defend itself against invasion by its U.S. "enemy" and claimed Sunday's launch involved a missile capable of carrying a nuclear warhead.
"The conversations that I have had. with my counterpart and in dealing with Beijing is that if (North Korea) did something else and if it looked to be long range, which this does, and if it looks like it is proactively leaning toward an ICBM, which it does, then we would take action", Haley said.
There were no signs that the attack sought to single out the USA or South Korea, despite its emergence at a time of high tensions on the Korean Peninsula.
Despite the push for a tougher stance, Haley held out the prospect of direct talks with North Korea, saying "we are willing to talk but not until we see a total stop of the nuclear process and of any test there".
The missile fell in the Sea of Japan outside of Japan's special economic zone.
On Monday, Neel Mehta, a researcher from Google, posted a message on Twitter which showed a sample of the code used in this weekend's attack. "It is similar to North Korea's backdoor malicious codes", said Choi.
Analysts said the test suggested an actual range of 4,500 kilometres (2,800 miles) or more if flown for maximum distance.More news: Putnam School Board and Budget Results
Noting that President Moon favors diplomatic engagement with the North, Hecker said it is imperative for Seoul and Washington to "craft a unified strategy on North Korea and speak with one voice".
Sunday's test came less than a week after South Korea elected a new president, Moon Jae-In, who advocates reconciliation with Pyongyang and had expressed a willingness to visit the North to ease tensions.
U.S. Ambassador to the United Nations Nikki Haley, right, is joined by Korean Ambassador Tae-yul Cho as she speak to reporters before a Security Council meeting on the situation in North Korea, Tuesday, May 16, 2017 at United Nations headquarters.
Haley said the US and China have been working on "a unified plan" on how to approach North Korea that would include stronger implementation of existing sanctions and tougher new sanctions.
Hong said South Korea had not yet received official word from the United States on whether Seoul should pay for an anti-missile USA radar system that has been deployed outside Seoul.
Matthew Pottinger, the Asia director on the National Security Council, met Chung Eui-yong, an adviser to the new South Korean government, and the two agreed to push for denuclearization through sanctions and dialogue, reports The New York Times.
"We compared the code samples between WannaCry and previous [Democratic People's Republic of Korea] activity, but the only similarities are public libraries", Burbage says.
Harris met Japanese Prime Minister Shinzo Abe, who also said China could apply more pressure to rein in North Korea.