VIENNAVIENNA (Reuters) - Iran has fired a diplomatic warning shot at Washington by raising the prospect of building nuclear reactors for ships while staying within the limits set by its atomic deal with major powers, a U.N. nuclear watchdog report showed on Thursday.

U.S. President Donald Trump has long railed against the 2015 nuclear deal for reasons including its limited duration and the fact it does not cover Iran's ballistic missile program. He has threatened to pull out unless European allies help "fix" the agreement with a follow-up accord.

Since Trump took office more than a year ago, Iran has stayed within limits on items including its stock of low-enriched uranium imposed by the deal, which also lifted painful international economic sanctions against the Islamic Republic.

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A quarterly report on Iran by the U.N. nuclear watchdog, which is policing the deal's restrictions, showed that Iran remained compliant, but also that it had informed the agency of a "decision that has been taken to construct naval nuclear propulsion in future".

Iran has raised that prospect in public statements before. In 2016, President Hassan Rouhani ordered the start of planning on the development of nuclear marine propulsion in reaction to what he called U.S. violations of the nuclear deal.

Rouhani was alluding to the lack of economic benefit to Iran from the deal because many companies including big Western banks continue to shun the country for fear of breaching separate U.S. financial sanctions that stayed in place after other sanctions were rescinded.

Analysts have said Iran is many years or decades away from having naval nuclear capacity. But mentioning it evokes both projecting military might and potentially enriching uranium beyond the limit of 3.67 percent purity imposed by the deal.

A senior diplomat said it was not clear from Iran's statement to the International Atomic Energy Agency (IAEA) whether it was referring to those previous statements, but they appeared to be a reaction to Trump's comments.

"Formally there is no clarity on this. But informally yes, why now? So obviously there is a link ... to the possibility that the JCPOA's future is questioned," the senior diplomat said, referring to the 2015 deal by its full name, the Joint Comprehensive Plan of Action.

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Iran's notification on marine propulsion could cover a range of intentions over any period of time, meaning there was no cause for concern, the senior diplomat said.

Iran has yet to respond to the IAEA's request for "further clarifications and amplifications", the report said, adding that if Iran had reached a concrete decision to build new facilities for marine propulsion it must provide design information.

The confidential quarterly IAEA report follows a statement by Iran's deputy foreign minister earlier on Thursday that Tehran will withdraw from the deal if there is no economic gains and major banks continue to stay away.