Satellite photos released on Monday appear to show China making progress on construction of at least two dozen hardened concrete hangars suitable for housing Chinese air force planes, including strategic bombers and inflight refuelers, on disputed islands in the South China Sea.

The photos were collected and studied by the Center for Strategic and International Studies (CSIS), a Washington-based think tank. They show construction work on man-made islands at Fiery Cross, Subi and Mischief reefs. The think tank reports the images were taken in June and July.

“These hangars are the smoking gun. You do not build nearly 80 hangars for civilian purposes on these tiny spits of land They're clearly meant for forward deployment of Chinese Air Force assets,” Greg Poling, Director of CSIS' Asia Maritime Transparency Initiative, told Fox News.

“They're holding a gun but they haven't put the bullet in it yet and they're saying that's not threatening,” he added.

China has said the new islands are primarily to assist fishermen and other causes, as well as to reinforce its sovereignty claims. China also says that the islands should be able to defend themselves, and that it is entitled to build whatever structures it wishes on them.

Meanwhile, the commander of the U.S. Pacific Fleet on Tuesday called for more military transparency to help calm simmering tensions in the region. Adm. Scott Swift also criticized China-Russia joint naval exercises planned next month in the South China Sea, saying the choice of location would not help with "increasing the stability within the region."

He also said any decision by China to declare an air defense identification zone over the strategic water body would be "very destabilizing from a military perspective."

The Hague-based Permanent Court of Arbitration issued a ruling on July 12 invalidating China's vast South China Sea maritime claims, a ruling that Beijing largely refused to accept. China strongly criticized the United States for encouraging the Philippines, a U.S. ally, to pursue the matter.

A satellite image showing development at Subi Reef. (CSIS/AMTI/DigitalGlobe)

Since then, Beijing has launched air patrols over the South China Sea, said it would consider declaring an air defense zone and vowed to continue work on man-made islands created from piling sand atop coral reefs in the highly contested Spratly group.

Swift was visiting the northern Chinese port of Qingdao as part of efforts to build trust and understanding between the two navies, now locked in a protracted competition for primacy in East Asia, where the U.S. has traditionally been the dominant military power.

Meanwhile, Japan protested Tuesday over a marked increase in the number of Chinese coast guard and fishing vessels in waters near disputed islands in the East China Sea.

A satellite image showing development at Mischief Reef. (CSIS/AMTI/DigitalGlobe)

Swift said the responses of all parties to the arbitration ruling had been consistent with their long-held positions and it was unclear what, if any, recent Chinese actions had been taken specifically in response.

"I think it's a mistake to take them individually and not look at them as a collective. And you have to look at it as an extension of an arc," Swift said.

Such judgments were made more difficult by a lack of transparency about intentions, he said, repeating a frequent U.S. criticism of China's secretive military.

"The uncertainty in the region is because of the lack of transparency and exactly where it is that arc is going. And that arc is defined by multiple data points," he said.

Swift cited two examples: The still unexplained cancellation by China of a visit by the aircraft carrier USS Stennis earlier this year, and the reason for the construction of the new aircraft hangers.

"That increases the angst and uncertainty, that lack of transparency, and that is generally destabilizing as opposed to a stabilizing action," Swift said.

The admiral said he was confident the U.S. Navy would continue to sail close to China's artificial islands in what are called freedom of navigation missions to reinforce the stipulations of the United Nations Convention on the Law of the Sea, although he said such decisions are made in Washington. China deeply resents such cruises, greeting them with threats and harassment.

Swift also criticized the planned China-Russia drills, saying, "There are other places those exercises could have been conducted."

"So I think that is a matter of concern and something that should be considered from the perspective of actions that are not increasing the stability within the region," he said.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.