Mr ElBaradei urged Iran to be as forthcoming as possible
Iran has come under more international pressure to respond to a proposal that it send uranium abroad for enrichment.
The head of the UN nuclear watchdog, Mohamed ElBaradei, said the draft deal offered last month was a "fleeting opportunity" to avoid confrontation.
Iran has raised "technical and economic considerations" with the International Atomic Energy Agency (IAEA) and has missed deadlines to respond.
US Secretary of State Hillary Clinton urged Iran to accept the draft plan.
She said that Iran was at a "pivotal moment".
"We urge Iran to accept the agreement as proposed. We are not changing it," she said, during a visit to Marrakesh, Morocco.
Britain, France and Russia have also called on Iran to promptly respond.
Under the plan brokered by the IAEA and agreed by Russia, the US and France, most of Iran's enriched uranium would be sent abroad to be turned into fuel rods for research use.
This is seen as a way for Iran to get the fuel it needs, while giving guarantees to the West that it will not be used for nuclear weapons.
Mr ElBaradei said that co-operation between Iran and Western powers was "primarily a matter of confidence building, which can only be achieved through dialogue."
He added: "I therefore urge Iran to be as forthcoming as possible in responding soon to my recent proposal."
The draft plan would require Iran to send about 1,200kg (2,600lb), or 70%, of its low-enriched uranium to Russia by the year's end for processing.
Subsequently, France would convert the uranium into fuel rods for use in a reactor in Tehran that produces medical isotopes.
Mr ElBaradei said the deal was a chance to "reverse course".
"The issue at stake remains that of mutual guarantees among the parties.
"I should add, however, that trust and confidence building are an incremental process that require focusing on the big picture and a willingness to take risks for peace.
"This is a unique and fleeting opportunity to reverse course from confrontation to co-operation and should therefore not be missed."
British Foreign Secretary David Miliband, speaking in Moscow after talks with his Russian counterpart, also appealed for a prompt answer from Tehran.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is converted into a gas by heating it to about 64C (147F)
Gas is fed through centrifuges, where its isotopes separate and the process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
"The Iranian government can be treated as a normal country on nuclear matters if it behaves like a normal country," he said.
Iran revealed the existence of a secret nuclear facility in September.
Iran says its nuclear programme is for purely peaceful purposes but the revelation of the existence of the new plant, near Qom, had increased fears in the West about Tehran's intentions.
French Foreign Minister Bernard Kouchner said the Western powers would not tolerate further delays by Iran.
Iran has said it wants the UN's nuclear watchdog to establish a committee to review a deal.
Iranian Foreign Minister Manouchehr Mottaki said the country had passed a request to the IAEA two days ago to establish a commission to review the deal.
In the meantime, Mr Mottaki said, Iran would "continue enrichment" for its nuclear needs.