As the board of governors of the International Atomic Energy Agency (IAEA) convened on Tuesday, Director General Mohamed ElBaradei said he hoped the conflict was merely a blip rather than a permanent rupture in talks with Tehran.
"I hope this is a hiccup in the process and not a permanent rupture," ElBaradei said.
He confirmed that Iran's uranium conversion work begun on Monday was "essentially the unravelling of the suspension" on fuel cycle work the country had agreed to in November to start talks with the European Union to guarantee it is not trying to develop nuclear weapons.
ElBaradei said the suspension was a voluntary decision.
"Nonetheless, it is essential for confidence-building and for resolution of outstanding issues relevant to Iran and past undeclared nuclear activities," he said.
ElBaradei appealed to both the Europeans and Iran to understand that the process is aimed at normalising a relationship that has been strained for the past quarter century.
"Iran must not be allowed to violate its international commitments"
US ambassador to the IAEA
"It will take time to build up confidence, to establish a normal relationship with Europe and ultimately with the United States," he said.
The US ambassador to the IAEA, Greg Schulte, said: "Iran must not be allowed to violate its international commitments and must not be allowed to develop nuclear weapons."
Schulte said the United States was consulting with its European allies and other board members on the next steps to take on the confrontation with Iran.
ElBaradei speaks to journalists
at a news conference in Vienna
Iran, which rejects charges that it is trying to develop nuclear weapons, on Monday resumed work on the nuclear fuel cycle that it had suspended last year in line with an international agreement.