ElBaradei, on his way to the Islamic republic, said the 35-member board of governors of the International Atomic Energy Agency (IAEA) was "impatient with Iran's cooperation."
The international probe into Iran's program "cannot go on forever. We have to discuss how to accelerate cooperation," he said. "We need to satisfy ourselves there are no undeclared nuclear activities in Iran."
The IAEA director general is due in Iran on Tuesday on the nuclear issue, although the Islamic republic insists it is not hiding any of its facilities from UN inspectors.
"We have a transparent and constructive cooperation with the agency, and this will continue," Iranian foreign ministry spokesman Hamid Reza Asefi told reporters on Sunday.
Iran also declared that its resumption of work on a key part of the nuclear fuel cycle was not a violation of its commitment to suspend uranium enrichment activities.
In a deal with the IAEA brokered last year by the European Union's big three -- Britain, France and Germany -- Tehran agreed to suspend uranium enrichment and related activities while UN inspectors delved into its program.
However, an IAEA board resolution on March 13 condemned Iran for failing to report sensitive nuclear activities, such as the possession of designs for sophisticated P-2 centrifuges that can make bomb-grade uranium.
Since then, ElBaradei said, Iran has delayed a crucial IAEA inspection mission to research the P-2 question.
"We were supposed to do the P-2 (investigation) last month and now we are going in on April 10," ElBaradei, making his third trip to Iran since February 2003, told reporters on a stopover in Frankfurt.
He said no date had yet to be set for Pakistan -- a nuclear power -- to allow IAEA inspectors in to the country to carry out so-called "environmental sampling" to compare certain key components with those sold on the international black market to Iran.
Iran has always claimed that the presence of highly enriched uraniumdiscovered by the IAEA was due to contamination from particles on the imported components.
HEU can be used both as nuclear fuel in civilian reactors or as the raw material for an atomic bomb.
IAEA inspectors have found traces of HEU at two sites in Iran. The United States says the particles are proof that Iran is trying to develop nuclear weapons, despite Iran's claims of contamination.
The IAEA board is to hear a report on Iran's nuclear program when it meets in Vienna in June to consider "progress in verifying Iran's declarations and of how to respond" to Iran's omissions in reporting on its atomic activities.
Asked if the delay in inspections would make it impossible to file a full report, ElBaradei said: "A month is still four weeks."
He said there has been "some slowing of cooperation" from Iran since it filed in October what it said was a full report on its nuclear activities.
The report also did not mention that Iran had the P-2 designs.
During his visit, ElBaradei will meet with Iranian President Mohammad Khatami, Foreign Minister Kamal Kharazi, Iran's top nuclear policy-maker Hassan Rowhani and the head of Iran's atomic energy organization Gholamreza Aghazadeh.
ElBaradei said he "would like to make clear in my visit that restoring and accelerating cooperation is in the interests of everybody."
"After all this time, there has been ample time for us to come to a conclusion," he said.