President Trump must certify to the US Congress whether Iran is in compliance with the agreement
The United States pressed for the International Atomic Energy Agency to carry out more nuclear inspections in Iran, warning that failure to do so would make the nuclear deal with Tehran "an empty promise."
US Ambassador to the United Nations, Nikki Haley, said that some countries were trying to shield Iran from more inspections by the IAEA, which is charged with verifying Tehran's compliance with the 2015 nuclear accord.
"Without inspections, the Iran deal is an empty promise," she said in a statement.
Haley's push for more inspections comes just 15 days before Trump must certify to the US Congress whether Iran is in compliance with the agreement.
"If the Iran nuclear deal is to have any meaning, the parties must have a common understanding of its terms," Haley said in a statement.
"Iranian officials have already said they will refuse to allow inspections at military sites, even though the IAEA says there must be no distinction between military and non-military sites.
"Now it appears that some countries are attempting to shield Iran from even more inspections."
Although she named no countries, diplomatic sources said she was referring to Russia.
Ten days ago, the head of the Iranian nuclear program, Ali Akbar Salehi, accused Washington of sabotaging the agreement and called on IAEA to resist Washington's "unacceptable demands."
He took particular aim at Haley, who he said had made unjustifiable demands regarding the verification of the nuclear accord. Those demands included IAEA inspections of Iranian military sites.
The United States has recently multiplied its attacks on the accord, which Trump had vowed to scrap last year during the US presidential campaign.
At a debate Wednesday organised by the Asia Society, Iranian Foreign Minister Javad Zarif, warning that a US withdrawal from the accord would be a "strategic mistake."
"The US needs to show it is a reliable partner," he said.
He also defended Iran's role in Syria and Iraq as justified by the need to defend populations threatened by terrorist organisations.
The Iranian nuclear accord, signed in July 2015 by Iran and six world powers - Germany, China, the United States, France, Britain and Russia - puts Iran's nuclear installations under strict surveillance.
The accord's aim is to guarantee that Iran's nuclear program is for peaceful purposes only, in exchange for a gradual easing of international sanctions.