ElBaradei, whose International Atomic Energy Agency (IAEA) inspectors were kicked out of North Korea in December 2002, said in an interview that Pyongyang's escalation by possibly testing an atomic bomb was "nuclear blackmail".
ElBarade said on Friday that North Korea needs "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," ElBaradei said on the sidelines of a non-proliferation conference at United Nations headquarters in New York.
ElBaradei said a North Korean nuclear test would have "disastrous political repercussions in Asia and the rest of the world."
He said "there could be a major environmental fallout, which again could lead to dissemination of radioactivity in the region".
North Korea is thought to have
one or two crude nuclear bombs
The IAEA verifies compliance with international safeguards mandated by the nuclear Non-Proliferation Treaty (NPT).
ElBaradei said nuclear tests by North Korea "would simply add insult to injury" it has already done to the world's non-proliferation regime.
The US government warned on Friday that any nuclear weapons test by North Korea would be a provocative act, as reports suggested an underground nuclear experiment could take place there.
The New York Times said in a report on Friday that US officials familiar with satellite and intelligence data thought North Korea was building a reviewing stand and filling in a tunnel, signs of a potential underground nuclear test.
"I don't want to get into discussing intelligence matters, but what I would say is that if North Korea did take such a step, that would just be another provocative act that would further isolate it from the international community," White House spokesman Scott McClellan said.
The United States, together with North Korea's neighbours, has been working through six-party talks to get Pyongyang to abandon its nuclear weapons ambitions.
"If North Korea did take such a step, that would just be another provocative act"
White House spokesman
The talks that group the two Koreas, Russia, China, Japan and the United States have been stalled for nearly a year since a third round of negotiations in June.
North Korea has boycotted the talks, citing "hostile" US policy, and has publicly announced it has nuclear weapons and could manufacture more of them.
ElBaradei said it was not too late for negotiations to work.
"Everybody knows what needs to be done," namely "to give North Korea security assurances" and "to respond to (their) economic and humanitarian needs" while Pyongyang must stop trying to acquire nuclear weapons, he said.
North Korea is thought to have one or two crude nuclear bombs, according to US intelligence reports.
International jitters were heightened on Sunday when North Korea test-fired a short-range missile, although US, South Korean and Japanese officials refused to link the incident to Pyongyang's drive for nuclear arms.