The replacement for António Guterres is due to be chosen next year and serve from 2027 to 2031. Argentina last month named its native son Grossi, 64, to fill the position and he is considered a top contender.
Since 2019, Grossi has led the International Atomic Energy Agency as it attempted to manage the still festering Iran nuclear dossier, which came to a head last year with surprise US and Israeli attacks on Iran's nuclear sites in a 12-day war in June.
Iran’s Ambassador and Permanent Representative to the United Nations, Amir Saeid Iravani appeared to single Grossi out without naming him in remarks to the Security Council on Wednesday.
“A candidate who has deliberately failed to uphold the UN Charter—or to condemn unlawful military attacks against safeguarded, peaceful nuclear facilities," he said, "undermines confidence in his ability to serve as a faithful guardian of the Charter and to discharge his duties independently, impartially and without political bias or fear of powerful States, as required under the Charter.”
Iran denies seeking a nuclear weapon but Israel and Western states doubt its intentions, especially after Grossi's IAEA flagged in the months running up to the conflict that Iran's enrichment activities were ramping up.
The country's uranium stock refined to up to 60% had hit nearly 275 kilograms, Grossi warned, which according to an IAEA yardstick was enough in principle for six nuclear bombs if enriched further.
No civilian purpose existed for such activities, Grossi warned.
The United States held five rounds of negotiations with Iran over its nuclear program earlier this year, for which US President Donald Trump set a 60-day deadline.
When no agreement was reached by the 61st day on June 13, Israel launched a surprise military offensive followed by US strikes on June 22 targeting key nuclear facilities in Isfahan, Natanz and Fordow.
The attacks killed nuclear scientists along with hundreds of military personnel and civilians. Iranian counterattacks killed 32 Israeli civilians and an off-duty soldier.
Grossi did not offer any assessment of the conflict but has worked to try to restore inspections of stricken sites in an effort Iran has largely opposed as diplomacy stays in a deep post-war freeze.
The United Nations has struggled to influence conflicts which have raged in the Middle East and Europe in recent years, earning the ire of both its critics and advocates who hope it can play a greater role in multilateral peace efforts.
Iran's intervention appeared to be the most substantive challenge yet to Grossi's bid.
Iravani said a UN secretary-general must have “a clear and non-derogable responsibility” to safeguard member states' rights and their equal participation in the global system.
“Failure to do so weakens the United Nations and erodes the principle of sovereign equality at the heart of the UN system,” he said.