Yukiya Amano, director general of the International Atomic Energy Agency, also said the IAEA remained more than £1 million short of the funds it needed for its extra work in verifying that Iran is living up to its part of the six-month accord reached in November.
He said: "The measures implemented by Iran, and the further commitments it has undertaken, represent a positive step forward, but much remains to be done to resolve all outstanding issues."
Iran struck a deal with the US, Britain, France, Russia, Germany and China to curb its nuclear programme in exchange for some easing of sanctions.
The accord, which took effect on January 20, was designed to buy time for negotiations on a final settlement of the decade-old stand-off over nuclear activity that Iran says is peaceful but the West fears may be directed towards developing a nuclear bomb capability.
Those talks began in Vienna last month and are due to resume on March 17 with the aim of getting a long-term deal by July.