"They are in a mourning period which I need to respect," International Atomic Energy Agency (IAEA) chief Grossi said in Helsinki, where he spoke at a nuclear conference.
"But once this is over, we are going to be engaging again," he said, describing it as a "temporary interruption that I hope will be over in a matter of days".
Grossi said the IAEA was planning to continue technical discussions with Iran but they had not yet taken place due to last weekend's helicopter crash that killed President Ebrahim Raisi and Foreign Minister Hossein Amirabdollahian.
The IAEA has been trying to expand its oversight of Iran's atomic activities while the country's uranium-enrichment programme continues to advance. Iran is enriching uranium to up to 60% purity, close to the 90% of weapons-grade, which no other country has done without developing nuclear weapons.
Tehran says its aims are entirely peaceful.
Iran currently has about 140 kg of uranium enriched to up to 60%, Grossi said. According to an IAEA definition, that is theoretically enough, if enriched further, for three nuclear bombs. The IAEA's last quarterly report in February said Iran had 121.5 kg, enough for two bombs.
Iran is still producing about nine kg a month of uranium enriched to up to 60%, Grossi said. It is also enriching to lower levels at which it has enough material for potentially more bombs.
Grossi, who two weeks ago said he wanted to start to see concrete results on improved cooperation from Iran soon, repeated that hope but said a more wide-ranging deal would require "a bit more time".
For now, his team had not made progress on the main issues, he said.
"It is high time there is some concrete issuance and if not resolution, some clarification of what is this," Grossi said of the uranium traces at undeclared sites.
"And I would say, confidence in many parts of the world (in Iran on the nuclear issue) is growing thinner.
