Diplomats say the UN will certify later this week that Iran's ability to make a nuclear bomb has been reduced because it has neutralised half of its material that can be turned quickly into weapons-grade uranium.
Two diplomats told the Associated Press news agency on Wednesday that a report by the IAEA, the UN nuclear agency, will say that Iran has diluted half of its stockpile of 20 percent enriched uranium, which could have been quickly further enriched to create fissile material for a bomb.
The move is part of Iran's commitments under a deal that mandates nuclear concessions by Tehran in exchange for a partial lifting of sanctions crippling its economy.
Meanwhile, Iran has formally protested against Washington's refusal to grant a visa to its new UN ambassador, saying the move damages international diplomacy and sets a "dangerous" precedent.
The US has said that Iran's selection of Hamid Aboutalebi to be its UN envoy is not acceptable.
Aboutalebi was involved in the takeover of the US embassy in Tehran which led to the US hostage crisis of 1979.
Aboutalebi has insisted his involvement in the group was limited to translation and negotiation.
Iran's UN Mission sent a delegation to meet the UN office of legal affairs on Tuesday about the issue, the UN spokesman Stephane Dujarric said.
Iran's letter said the US was breaching its obligations under the US-UN host country agreement, which is a treaty and US law that generally requires the host country to allow access to diplomats and UN guest speakers.
"The agreement has unambiguously stated that its provisions shall be 'applicable irrespective of the relations existing between the governments of the persons referred to in that section and the government of the United States'," the letter said.
"This decision of the US government has indeed negative implications for multilateral diplomacy and will create a dangerous precedence."