International atomic monitors in Iran last week detected uranium enriched to levels just below that needed for a nuclear weapon, according to two senior diplomats, underscoring the risk that the country’s unrestrained atomic activities could prompt a new crisis.

The International Atomic Energy Agency is trying to clarify how Iran accumulated uranium enriched to 84 percent purity — the highest level found by inspectors in the country to date, and a concentration just 6 percent below what’s needed for a weapon. Iran had previously told the IAEA that its centrifuges were configured to enrich uranium to a 60 percent level of purity.

Inspectors need to determine whether Iran intentionally produced the material, or whether the concentration was an unintended accumulation within the network of pipes connecting the hundreds of fast-spinning centrifuges used to separate the isotopes. It’s the second time this month that monitors have detected suspicious enrichment-related activities.

The development comes as Iran is increasingly isolated from the West and nuclear talks with world powers remain suspended. The country has also faced widespread condemnation for its deadly crackdown on major protests and the US and European Union have tightened sanctions on Iran over its military support for Russia’s war on Ukraine.

Earlier on Sunday, Israel blamed Iran for a Feb. 10 attack on an oil tanker in the Arabian Sea. The incident came about a fortnight after a drone strike on a weapons depot near Iran’s city of Isfahan that Tehran blamed on Israel.

The IAEA is preparing its quarterly Iran safeguards report ahead of a March 6 Board of Governors meeting in Vienna, where the Gulf nation’s nuclear work will figure prominently on the agenda.

Iran hasn’t submitted required forms declaring its intention to raise uranium enrichment levels at two facilities near the towns of Natanz and Fordow, according to one diplomat.

Even if the detected material was mistakenly accumulated because of technical difficulties in operating the centrifuge cascades — something that has happened before — it underscores the danger of Iran’s decision to produce highly enriched uranium, the other diplomat said.

The IAEA has repeatedly said levels even at just 60 percent are technically indistinguishable from the level needed for a nuclear weapon. Most nuclear power reactors use material enriched to 5 percent purity.

A nuclear deal between Iran and world powers unraveled after then-President Donald Trump withdrew the US in 2018 and reimposed sanctions. In response, Iranian officials expanded the country’s nuclear program. Tehran denies it’s seeking to build atomic warheads but concerns it might develop the technology to do so propelled years of diplomacy that led to the deal with world powers.

IAEA Director General Rafael Mariano Grossi called the atomic deal an “empty shell last month and said Iran has sufficient nuclear material for several weapons, should it make the political decision to move forward.