US Secretary of State John Kerry has warned North Korea it would defend its allies after holding talks in the South Korean capital Seoul.
Speaking at a joint news conference with South Korean counterpart, Foreign Minister Yun Byung Se, Mr Kerry said the US would never accept North Korea as a nuclear power and described rhetoric from Pyongyang as "unacceptable".
He added that a missile test would be another "unwanted contribution to an already volatile situation" and a "huge mistake".
He said: "It would indicate who was being provactive with an exclamation point again.
"We will defend our allies. We will stand with South Korea, Japan and others. We will defend ourselves.
"Kim Jong Un needs to understand, as I think he probably does, what the outcome of a conflict would be."
Intelligence reports from the Japanese, South Koreans and Americans have indicated that a North Korean missile test could take place at any time, though there has been silence from the leadership in Pyongyang.
The focus in the North Korean capital has been on a weekend of celebrations to mark a year in office for Kim Jong Un, which fell yesterday, and the anniversary of Mr Kim's grandfather, Kim Il Sung, the founder of the nation.
The level of rhetoric to emerge from North Korea is unprecedented.
Over several weeks, the regime has declared itself to be in a "state of war" with the South, announced that a mothballed nuclear site is to be reopened and threatened to carry out nuclear attacks against the US.
Mr Kerry arrived in the region as confusion surfaced in Washington over the true status of North Korea's nuclear capability.
The broad consensus is that while Kim Jong Un does possess nuclear devices and has crossed the "nuclear threshold", he does not have the capability to launch a nuclear missile.
However, at a congressional hearing on Thursday night, it emerged that one US government agency believes that Kim Jong Un does have nuclear weapons which could be placed inside a ballistic missile and fired.
Republican US Representative Doug Lamborn, quoting from a March 2013 DIA report which was inadvertently labelled "unclassified", said: "(The) Defence Intelligence Agency (DIA) assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles, however the reliability will be low."
The Pentagon was quick to issue a written clarification on the matter.
Spokesman George Little said: "In today's House Armed Services Committee hearing on the Department of Defence budget, a member of the committee read an unclassified passage in a classified report on North Korea's nuclear capabilities.
"While I cannot speak to all the details of a report that is classified in its entirety, it would be inaccurate to suggest that the North Korean regime has fully tested, developed, or demonstrated the kinds of nuclear capabilities referenced in the passage."
Washington added it was concerned about unexpected developments linked to the inexperience of 30-year-old Kim Jong Un.
One official said: "Kim Jong Un's youth and inexperience make him very vulnerable to miscalculation. Our greatest concern is a miscalculation and where that may lead.
"We have seen no indications of massive troop movements, or troops massing on the border, or massive exercises or anything like that that would back up any of the rhetoric that is going on."
North Korea has said that it does possess advanced nuclear devices.
President Barack Obama, speaking after White House talks with UN Secretary General Ban Ki Moon, said "nobody wants to see a conflict".
He added: "We both agree that now is the time for North Korea to end the kind of belligerent approach that they've been taking.
"It's important for North Korea, like every other country in the world, to observe basic rules and norms."
This whole crisis stems from Pyongyang's desire to pursue a nuclear programme which it says it needs to defend itself from "American aggression".
By manufacturing this crisis, Kim Jong Un is likely to be demonstrating strength domestically and thus bolstering his legitimacy.
Internationally, he is determined that his country is taken seriously as a nuclear power.
He would want an acceptance from the Americans that he is part of the 'nuclear club' as a pretext to any negotiations to end this crisis.