The size of Iran’s most contested uranium stockpile has declined significantly for the first time in four years following a landmark nuclear deal with world powers in November, the U.N. atomic agency reported Thursday.

As a result, Iran’s holding of uranium gas enriched to a fissile purity of 20 percent — a relatively short technical step from the level required for nuclear weapons — is now well below the amount needed for processing into a bomb.

The stockpile is closely watched. Israel, believed to be the Middle East’s only nuclear-armed power, warned in 2012 that if Iran amassed enough such refined uranium for a single bomb it would be a “red line” for possible military action.

Iran agreed under a Nov. 24 deal with six big powers to stop its 20-percent enrichment, which it began in 2010, and has since diluted some of the material to a lower concentration and converted some into less proliferation-prone uranium oxide.

“That decrease has been quite important,” a senior diplomat familiar with Iran’s nuclear program said. “That progress has been quite substantial in terms of inventory.”

Thursday’s report by the International Atomic Energy Agency (IAEA) also showed that Iran was meeting its commitments under last year’s interim agreement with the six powers to curb its most sensitive nuclear work in exchange for some easing of sanctions.

“Things are progressing as planned,” the diplomat said.

The IAEA report was issued to member states just hours after Iran and the six countries — the United States, France, Germany, Britain, China and Russia — ended a first round of negotiations in Vienna aimed at a final settlement of the decade-old dispute over the nature of Tehran’s nuclear activity. The next round was set for March 17.

U.N. nuclear inspectors are playing a critical role in monitoring that Iran is living up to its side of last year’s six-month accord, designed to buy time for the negotiations on a comprehensive agreement over atomic activity that Tehran says is entirely peaceful but which the West fears may have military designs.

Iran’s reserve of 20-percent uranium fell to 354 pounds in February from about 431 pounds in November, the IAEA said. About 550 pounds is needed for the core of one nuclear warhead, experts say.