WASHINGTON — With only one month left before a deadline to complete a nuclear deal with Iran, international inspectors have reported that Tehran’s stockpile of nuclear fuel increased about 20 percent over the last 18 months of negotiations, partially undercutting the Obama administration’s contention that the Iranian program had been “frozen” during that period.

But Western officials and experts cannot quite figure out why. One possibility is that Iran has run into technical problems that have kept it from converting some of its enriched uranium into fuel rods for reactors, which would make the material essentially unusable for weapons. Another is that it is increasing its stockpile to give it an edge if the negotiations fail.

The extent to which Iran’s stockpile has increased was documented in a report issued Friday by the International Atomic Energy Agency, the United Nations organization that monitors compliance with nuclear treaties. The agency’s inspectors, who have had almost daily access to most of Iran’s nuclear production facilities, reported finding no evidence that Iran was racing toward a nuclear weapon, and said Tehran had halted work on facilities that could have given it bomb-making capabilities.

The overall increase in Iran’s stockpile poses a major diplomatic and political challenge for President Obama and Secretary of State John Kerry, who flew back to the United States from Geneva on Monday for treatment of a broken leg he suffered in a bicycling accident, as they enter a 30-day push to try to complete an agreement by the end of June. In essence, the administration will have to convince Congress and America’s allies that Iran will shrink its stockpile by 96 percent in a matter of months after a deal is signed, even while it continues to produce new material and has demonstrated little success in reducing its current stockpile.