The world must show zero tolerance to North Korea and put pressure on it not to launch a nuclear test, the head of the UN's nuclear watchdog has said.
There are fears North Korea may be about to test a nuclear bomb
Mohamed ElBaradei said a test would have "disastrous political and environmental consequences".
The warning came amid fresh reports of intelligence suggesting North Korea may be preparing to test a nuclear weapon for the first time.
Earlier Japan urged North Korea to return to six-party nuclear talks.
Pyongyang has shunned multilateral discussion of its nuclear programme for almost a year.
Japanese Foreign Minister Nobutaka Machimura said: "If there is no progress, we have to think of other options, such as taking this matter to the United Nations Security Council."
The Security Council could impose international sanctions on North Korea.
The New York Times reported on Friday that US officials were assessing satellite photographs that appeared to show extensive preparations for a test.
But the officials warned it could simply be a ruse by North Korea to strengthen its bargaining power with Washington.
Mr ElBaradei called Pyongyang's apparent escalation "nuclear blackmail".
He said North Korea needed "to understand that the international community has zero tolerance for any new country to go for a nuclear weapon".
"I hope they will not test. I hope every leader who has contact with North Korea is on the phone today with North Korean authorities to dissuade them from testing," he said.
"There could be a major environmental fallout which again could lead to dissemination of radioactivity in the region."
"So enough rituals. Now the time has come after 12 years since we reported North Korea in non-compliance to the Security Council... [for all concerned parties] urgently to bite the bullet and find a comprehensive solution and avoid this escalating nuclear danger."
Mr ElBaradei was speaking to reporters on the sidelines of the five-yearly review of the Nuclear Non-Proliferation Treaty, which is aimed at stopping the spread of nuclear weapons.
Two years ago, North Korea became the first country to pull out of the treaty, under which it had pledged not attempt to procure nuclear arms.
There has been an on-going debate about whether the country has managed to turn its stockpile of nuclear fuel into warheads.
Earlier this year, Pyongyang said it had manufactured a bomb, but this has not been independently verified.