After five hours of intensive negotiations with Iran's nuclear point-man -- national security chief Hassan Rowhani -- ElBaradei announced Iran had pledged to file a new report on its nuclear activities before the International Atomic Energy Agency (IAEA) next meets in June.
ElBaradei said Iran had agreed to a tough action plan and timetable aimed at clearing up serious questions over its bid to generate atomic power -- seen by the United States and Israel as a convenient cover for weapons development.
"Today I was reassured by Dr. Rowhani that Iran is committed to continue to actively cooperate," said ElBaradei.
"I can tell you that I am quite satisfied," ElBaradei added, saying his talks had yielded "welcome and positive steps".
But the United States sniffed at Iran's promises, noting that the Islamic republic had reneged on similar vows in the past.
State Department deputy spokesman Adam Ereli said in Washington: "It's great if they actually live up to their promise, but so far they haven't done that."
Last October, Iran gave the IAEA what it said was a complete declaration of its nuclear activities.
It was later found to have made a number of omissions, including its acquisition of designs for sophisticated P-2 centrifuges that can produce weapons-grade uranium, way above the level of enrichment required for atomic reactors.
In December, it signed the additional protocol to the nuclear Non-Proliferation Treaty (NPT), which sets tougher conditions for IAEA inspections and commits the country to filing a new and more detailed declaration to the Vienna-based agency within six months.
"Dr. Rowhani assured me that we will get some important information before the end of this month and that we would also hope to get information under the additional protocol by mid-May," ElBaradei said.
Sources close to the talks said the main outstanding issues were the P-2 centrifuge and accounting for traces of highly enriched uranium, which could be weapons-grade, found by the IAEA at two sites here.
Iran says the traces came into the country on equipment bought on the black market from Pakistan. To verify this, ElBaradei said he hoped Pakistan would allow the IAEA to take so-called "environmental samples" there.
An IAEA official said Iran had promised to give a better accounting of where it has stored equipment, which could explain where traces of highly enriched uranium have been found.
But Iran was still balking over certain issues, such as how much work it should be allowed to conduct on the nuclear fuel cycle while the complex IAEA investigation is still running.
Iran's atomic energy chief, Gholam Reza Aghazadeh, said negotiators had rejected a call from ElBaradei to delay resumption of uranium conversion work -- or producing the precursors for the enrichment process -- at a facility in the central city of Isfahan.
Iran insists such work does not violate its suspension of enrichment-related activities.
And while Aghazadeh told reporters Iran would stop the construction and assembly of centrifuges used to enrich uranium as of April 9, it was a repeat of a promise made earlier this year.
ElBaradei said a team of IAEA inspectors would arrive in Iran on April 12 to verify the suspension of uranium enrichment was being respected.
Foreign Minister Kamal Kharazi said Iran expected the IAEA investigation to end in June.
"The deadline to us is June," he told reporters.
Rowhani said Iran would "react firmly" if, after the IAEA meeting in June, the Iran dossier still looked as if it was a long way from being settled.
"But personally I do not think Iran will leave the NPT," he said.
If Iran is judged to have failed to meet IAEA demands, the IAEA board could declare the country in breach of the NPT and refer the matter to the matter to the UN Security Council, which could choose to impose sweeping sanctions.
But with a new declaration to verify and more environmental samples to examine, analysts say the fruits of ElBaradei's latest visit are likely to remain unclear until after the June meeting.