The U.S. military build-up in the Middle East means Iran’s window to reach a diplomatic agreement over its atomic activities is at risk of closing, according to the head of the United Nations nuclear watchdog.

The International Atomic Energy Agency has discussed concrete proposals with Iranian Foreign Minister Abbas Araghchi to inspect sites bombed last year by Israel and the U.S., Director General Rafael Mariano Grossi told Bloomberg Television on Thursday. Whether those plans can be finalized urgently is an open question.

“There is not much time but we are working on something concrete,” said Grossi, referring to six hours of meetings earlier this week in Geneva with Iranian diplomats. “There are a couple of solutions the IAEA has proposed.”

IAEA inspectors haven’t verified the state of Iran’s stockpile of near-bomb-grade uranium or assessed the scope of damage dealt to enrichment facilities for more than eight months. Before the Israeli attacks in June, Iran had enough highly-enriched material to quickly craft about a dozen warheads, if orders were given to weaponize its nuclear program.

Grossi said he also met with U.S. President Donald Trump’s envoys on Tuesday in Geneva. The White House has repeatedly asserted the U.S. “obliterated” Iran’s atomic work in last year’s strikes but has threatened the Islamic Republic with further military action if it doesn’t agree to a new deal.

An IAEA return to the damaged facilities in Fordow, Isfahan and Natanz “hinges on the possibility of a wider type of agreement,” Grossi said. “We are conscious of the fact that there is this political negotiation.”

Following talks held earlier this week, Iran and the U.S. expressed cautious optimism for a diplomatic outcome. Iran said it reached a “general agreement” around the principle of nuclear restrictions in exchange for sanctions relief. Both sides committed to drafting and exchanging proposals before meeting again.

In the meantime, the U.S. has continued to accumulate military assets in the region, while Iran conducted naval drills with Russia that temporarily restricted some shipments through the Strait of Hormuz — a key choke point for energy exports.

At the heart of the nuclear dilemma is the issue of uranium enrichment — the tightly-guarded process of separating uranium isotopes which can be used to fuel nuclear reactors or bombs. Israel and the U.S. want to cease all enrichment activity and dismantle plants. Iran insists on retaining some fuelmaking capacity for peaceful purposes.

During his first term, Trump walked away from an agreement which capped Iranian enrichment activities and imposed IAEA inspections in return for sanctions relief. The Islamic Republic subsequently expanded its program, accumulating tons of enriched uranium until its nuclear sites were bombed during last year’s 12-day war.

“There cannot be a deal if the IAEA isn’t able to verify,” said Grossi, who added he’s trying to find a solution by threading the red lines set by both sides.

“It’s not impossible,” he said. “There are certain things that Iran understands cannot be pursued. We have to provide the watertight verification there is no deviation.”

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