VIENNA (Reuters) - Iran told the U.N. nuclear watchdog on Tuesday it would ship 20 tonnes of heavy water abroad to avoid breaching a limit on its stock of that substance under a landmark deal with six world powers, officials said.

Heavy water, a moderator used in a type of reactor that can produce plutonium, is not the most sensitive part of Iran’s nuclear program. But Tehran’s stock of it is restricted to 130 tonnes under its 2015 deal with the major powers.

Iran has already breached that limit twice since the deal imposed restrictions on its nuclear activities in January last year, when sanctions against Tehran were also lifted under the agreement.

U.S. President Donald Trump has called the agreement “the worst deal ever negotiated” and Washington strongly criticized Iran when it breached its heavy water limit last year.

The deal calls for Iran’s excess heavy water to be sold to a foreign buyer. Washington and its allies have accepted that it be shipped to Oman while a buyer is sought, which is what Iran did last year.

“Twenty metric tonnes of its stock of heavy water will be transferred out of the country,” said one official who attended an International Atomic Energy Agency (IAEA) meeting at which the U.N. watchdog said Tehran had informed it of the plan on Tuesday.

The IAEA did not specify where the heavy water would be transferred to or when that would happen, but it said it would be soon, three officials who attended the meeting said on condition of anonymity because they were not authorized to discuss the matter publicly. There was no mention of a buyer.

A quarterly IAEA report on Iran on Friday said Tehran’s stock of heavy water had reached 128.2 tonnes, which a senior diplomat described as “very high”.

Iran’s heavy water production plant has been closed for maintenance since May 16. It is not clear when it will restart, but once it does Iran could reach the 130 tonne limit within about six weeks, the senior official said on Friday.

“For sure, when it comes back online something needs to be done within a few weeks’ time (to stay within the cap),” he said.

That now appears to be happening, the officials said after Tuesday’s briefing by the IAEA, which is in charge of policing the restrictions the deal placed on Iran’s nuclear activities.

The United States said in April that Iran was in compliance with the deal but it has also launched a wider policy review on how to deal with the Islamic Republic.