UN nuclear watchdog 'seriously concerned' over Iran blocking of site inspections
The Vienna-based International Atomic Energy Agency (IAEA) is expected to discuss a report issued last month which flagged up the sites. If they pass a resolution critical of Iran, it would be the first of its kind since 2012.
Even though the two sites are not thought to be key to Iran's current activities, the agency says it needs to know if past activities going back almost two decades have been properly declared and all materials accounted for.
Iran told the agency the report was a source of "deep regret and disappointment" and hinted the queries were based on "fabricated information" from "intelligence services".
Israel has previously claimed its intelligence services unearthed information on an alleged previous nuclear weapons programme in Iran and Western states have voiced concern over Iran's denial of access to the sites, with the United States being particularly vocal.
The latest row over access comes as a landmark deal between Iran and world powers in 2015 continues to unravel.
Under the deal, known as the Joint Comprehensive Plan of Action (JCPOA), Iran committed to curtailing its nuclear activities in return for sanctions relief and other benefits.
But the country has slowly abandoned its commitments in retaliation for US President Donald Trump's decision two years ago to renounce the deal and reimpose severe sanctions.
Iran's stockpile of enriched uranium is now almost eight times the limit fixed in the accord, according to an IAEA assessment published earlier this month.
However, the level of enrichment is still far below what would be needed for a nuclear weapon.
The IAEA says it continues to have access to all the facilities needed to monitor Iran's current nuclear activity.
The latest tension will further complicate efforts by the deal's EU signatories - the so-called E3 of France, Germany and Britain - to keep the deal from collapsing.
Trump has called for the E3 to follow his lead and leave the deal.
Last month, the US said it was ending sanctions waivers for nations that remain in the Iran nuclear accord - a move likely to have most impact on Russian firms working on Iran's nuclear programme.
The American move brought condemnation from the E3 and was branded "unlawful" by Tehran.
Iran is also concerned that the US is pushing for an extension to an international arms embargo against Tehran which is set to be progressively eased from October.
Last week Iranian President Hassan Rouhani urged other UN Security Council members, especially veto-wielding China and Russia, to stand against the American "conspiracy".