Russia has added troops, aircraft, and weapons to Ukraine's Crimean peninsula in what amounts to a "significant" buildup of forces over the past 18 months, according to U.S. intelligence officials, observers, and new satellite photos that reveal the locations of new S-400 air defense systems and improvements to Soviet-era bases.

Those officials and observers of the region say the additional firepower gives Moscow greater defensive control over the Black Sea and puts offensive fighters and ships closer to the Middle East, Defense One reports.

The photos, taken between January 2018 and April 2019 by private satellite imaging company Planet Labs and provided to Defense One, show five S-400 batteries, five S-300 air-defense systems, and fighter jets at four locations. They also show improvements to Soviet-era military installations.

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Observers said the development likely means that Moscow has no near-term intention of returning the Ukrainian territory it seized in 2014, which the United States has said is required before it will resume normalized relations.

Instead, that buildup "suggests that Russia is interested in being able to exercise more control over the Black Sea, which then affords them the ability to project power beyond their immediate environment," said Sarah Bidgood, the director of the Eurasia Nonproliferation Program at Middlebury College’s James Martin Center for Nonproliferation Studies. "This is a significant buildup. NATO is going to be under increasing pressure from allies in the region to show that it’s able to push back against Russian attempts to gain greater control of the Black Sea."

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The Russian military now has 81 airplanes and helicopters in Crimea. "The combat radius covers all Ukraine and beyond the Black Sea. It significantly increases their strike options, potentially extending to the Middle East," the official said.

Russia’s Black Sea Fleet recently added 10 warships that can launch the Kalibr cruise missile: six-diesel electric Kilo-class attack submarines and four surface ships, the official said. The current versions of the Kalibr can hit targets up to 1,500 miles away; and Russia claims to be working on a new variant with a range of 2,800 miles.

A U.S. intelligence official said the Kalibrs will allow the fleet "to strike targets beyond the Black Sea, including southern Europe and Syria, without even departing Sevastopol."

Earlier, Head of NATO Representation to Ukraine Alexander Vinnikov said "Russia's ongoing militarization of Crimea, the Black Sea and the Sea of Azov poses further threats to Ukraine’s independence and undermines the stability of the broader region."