He said the International Atomic Energy Agency's (IAEA) discovery in January of designs for sophisticated P2 centrifuges in Iran for making highly enriched uranium (HEU) that could be used to make the bomb was "a setback, a great setback" since Iran had claimed in October to have fully disclosed its nuclear activities.
This led to a tough resolution against Iran at an IAEA board of governors meeting on Saturday in Vienna.
And that drew Iran's ire and led Tehran to suspend inspections that had been scheduled for last week.
But the Iranians would "be punishing themselves if they delay things," ElBaradei told reporters on a flight from Vienna to Washington, where he is to meet on Wednesday with US President George W. Bush.
The IAEA, which verifies the nuclear Non-Proliferation Treaty (NPT), has since February 2003 been working to determine whether Iran's nuclear programme is peaceful, or geared towards secretly developing atomic weapons as the United States has charged.
It is to review the Iranian programme in June and Tehran's decision to put off the inspection from last week until the end of April could mean the inspectors would not have enough time to file a full report, a diplomat said in Vienna.
ElBaradei said: "The best way obviously to answer all these suspicisons is for us to go back as early as possible."
He said the inspection was a "technical visit ... to clarify some of issues around the P2 and going to the pilot plant (for making enriched uranium) at Natanz (in southern Iraq) to make sure it is locked. it is sealed, it is not operational."
Iran had pledged in October to suspend the enrichment of uranium as a confidence-building measure with the IAEA.
ElBaradei said it was not good for the IAEA inspectors to change their schedules and "definitely inspections should not be dependent on holidays," as Iran claimed in putting off the inspection because of the Iranian New Year next week.
He said he expected Iran to over the next few days reverse its suspension and allow the inspectors in.
ElBaradei said it was a lesson for Iran that "at this stage people are hypersensitive and they (Iran) need to show that there is absolutely 100 percent full transparency."
Iran must make a full declaration on its nuclear program soon, he said.
"I told the Iranians, if there's anything still to be declared under the (international) safeguards agreement under the (additional) protocol (to the , they should come with it yesterday and not tomorrow," ElBaradei said.
Iran's ambassador to the IAEA Pirooz Hosseini said in Vienna last week that Tehran had not told all in October "because what we announced at that time was based on our obligations under the safeguard agreement" of the NPT.
Iran then in December signed the additional protocol allowing for wider inspections and Hosseini said that "according to the time line of the additional protocol, we are going to provide every information which is necessary for the agency."
ElBaradei said if Iran wanted things to move forward at the IAEA board meeting in June "they have to come with the protocol as early as possible for us to do the proper analysis and to report on it.
"If it comes one day before the June board, then we cannot digest it," ElBaradei said.
He said the IAEA hopes to wrap up its investigation of Iran's nuclear program for suspected weapons-related activities by the end of this year.
"If there is no smoking gun, no 800-pound gorilla, if we get cooperation from the country of origin (for imported nuclear equipment), I see no reason why we should not be able to have at least most of it wrapped up by the end of the year," he said.
The major remaining issue is whether traces of highly enriched uranium that could be weapon-grade found by the IAEA at two sites in Iran were homemade or came from contamination from centrifuge parts bought on a black market abroad, as Iran claims.
"We need all the support from Iran (to resolve this) but also need support from the importing country," ElBaradei said, referring to environmental samples the IAEA wants to make in Pakistan on centrifuge parts there.