WASHINGTON/DUBAI (Reuters) - Recent satellite imagery shows major expansions at two key Iranian ballistic missile facilities that two American researchers assessed are for boosting missile production, a conclusion confirmed by three senior Iranian officials.

The enlargement of the sites follows an October 2022 deal in which Iran agreed to provide missiles to Russia, which has been seeking them for its war against Ukraine. Tehran also supplies missiles to Yemen's Houthi rebels and the Lebanese militia Hezbollah, both members of the Iran-backed Axis of Resistance against Israel, according to U.S. officials.

Images taken by commercial satellite firm Planet Labs of the Modarres military base in March and the Khojir missile production complex in April show more than 30 new buildings at the two sites, both of which are located near Tehran.

The images, reviewed by Reuters, show many of the structures are surrounded by large dirt berms. Such earthworks are associated with missile production and are designed to stop a blast in one building from detonating highly combustible materials in nearby structures, said Jeffrey Lewis of the Middlebury Institute of International Studies at Monterey.

The expansions began at Khojir in August last year and at Modarres in October, Lewis said, based on images of the sites.    

Iran's arsenal is already the largest in the Middle East, estimated at more than 3,000 missiles, including models designed to carry conventional and nuclear warheads, experts say.

Three Iranian officials, who asked not to be identified because they were not authorized to speak publicly, confirmed that Modarres and Khojir are being expanded to boost production of conventional ballistic missiles. 

"Why shouldn't we?" said one official. 

A second Iranian official said some of the new buildings would also allow a doubling of drone manufacturing. Drones and missile components would be sold to Russia, drones would be provided to the Houthis and missiles to Hezbollah, the source added.

Reuters was unable independently to confirm the Iranian officials' comments.

Iran's mission to the United Nations did not respond to a Reuters request for comment on the expansion of the complexes. Tehran has previously denied providing drones and missiles to Russia and the Houthis. Hezbollah's media office did not immediately respond to requests for comment.

Houthi spokesperson Mohammed Abdulsalam said a boost in Iran's weapons manufacturing would not have any impact in Yemen because the Houthis develop and manufacture aircraft independent of Iran. 

Lewis analyzed the Planet Labs imagery with Decker Eveleth, an associate research analyst at CNA, a Washington thinktank, as part of a Middlebury project that monitors Iranian missile infrastructure.

"We know that Russia is on the hunt for low-cost missile capabilities, and it has gone to Iran and North Korea," said Lewis. 

Moscow and Pyongyang have denied the transfer of North Korean missiles to Russia. The Russian embassy in Washington and North Korea's mission to the United Nations did not immediately respond to requests for comment for this story.

The two U.S. researchers said in separate interviews that it was not clear from the photos what kinds of missiles would be produced at the new facilities, which still appeared to be under construction.

Any increase in Tehran's missile or drone production would be concerning to the United States, which has said that Iranian drones help sustain Russia's assault on Ukrainian cities, and to Israel as it fends off attacks from Iran-backed groups, including Hezbollah.

The U.S. Office of the Director of National Intelligence declined to comment on the reseachers' analysis. 

A U.S. National Security Council spokesperson declined to confirm their assessment, adding that the United States has implemented various measures, including sanctions, intended to constrain Iranian missile and drone production and exports.

 Reuters in February reported that Iran had sent surface-to-surface ballistic missiles to Russia for use against Ukraine. Iran denied providing the weapons. Washington said it could not confirm the transfers but it assumed Tehran intended to provide missiles to Moscow.

NEW BUILDINGS, DIRT BERMS    

Shahid Modarres and Khojir are overseen by the Islamic Revolutionary Guard Corps (IRGC), the elite paramilitary organization that plays a central role in Iran's missile and nuclear programs. It controls large segments of the Iranian economy and answers directly to Iran's supreme leader, Ayatollah Ali Khamenei.

The complexes have long been associated with the development and production of Iran's short- and medium-range ballistic missiles and rockets for the country's space program.

On Nov. 12, 2011, a massive explosion destroyed a large swath of Shadid Modarres associated with solid fuel missiles, killing 17 IRGC officers. They included Gen. Hassan Moqaddam, regarded by Iran as the "architect" of its ballistic missile program.

Construction at Shahid Modarres, which began again after the 2011 explosion, accelerated last year, the second Iranian official said.

"I think the Iranians may have chosen not to berm the buildings (before the explosion) because they didn't want to draw attention to them," said Lewis. "They learned the hard way."

Eveleth and Lewis said the sites' long history with Iran's missile program - Shahid Modarres is considered by some experts as its birthplace - and the numerous dirt berms support their assessment that Tehran is expanding ballistic missile production.

"When we see where you basically have an entire production line that is bermed like that, that's usually missiles," said Eveleth.

(Reporting by Jonathan Landay and Parisa Hafezi; Editing by Don Durfee, Cynthia Osterman and Daniel Flynn)

By Jonathan Landay and Parisa Hafezi