Grossi's visit came at a time of heightened regional tensions and with his International Atomic Energy Agency (IAEA) criticizing Iran for lack of cooperation on inspections and other outstanding issues.
"The present state is completely unsatisfactory for me. We are almost at an impasse and this needs to be changed," Grossi told reporters at the airport in Vienna, where the IAEA is based, as quoted by AFP.
He said there was no "magic wand" to solve a "very, very complex set of issues", while he pressed the Islamic Republic to "deliver very soon".
"But of course, for me and also I would say for the international community, there is a need to have some results sooner rather than later," he said.
The IAEA has long sought answers from Iran about two sites near Tehran that inspectors say bore traces of man-made uranium.
In addition, the IAEA has found that, between June and November last year, Iran slowed down the enrichment to 3 kg per month, but that jumped back up to a rate of 9 kg at the end of the year.
The increase came soon after Tehran barred a third of the IAEA's core inspections team, including the most experienced, from taking part in agreed monitoring of the enrichment process.
Grossi said in February that Iran continues to enrich uranium well beyond the needs for commercial nuclear use despite UN pressure to stop it.
Grossi said on Tuesday that a March 2023 agreement with Iran was "still valid" but required more "substance".
The agreement was reached during Grossi's last visit to Iran and outlined basic cooperation measures including on safeguards and monitoring.
The IAEA chief said, however, that there had been a "slowdown" in the agreement's implementation, including Iran reducing the number of inspections and withdrawing the accreditation of a group of IAEA experts.
