This April 2 satellite image reportedly shows airstrip construction on Fiery Cross Reef in the South China Sea. (Handout/Reuters)

THE UNITED States last week stepped up its efforts to call attention to China’s massive and provocative expansion of infrastructure in a disputed portion of the South China Sea. The Navy invited CNN aboard a surveillance flight and released video the next day of what one U.S. official called an attempt “to make sovereign land out of sand castles” in the Spratly Islands. The flight by a P-8A Poseidon aircraft drew eight warnings from the Chinese navy and an angry denunciation by the Chinese Foreign Ministry, which called it “very irresponsible and also very dangerous.” But the U.S. action was legal and appropriate.

China is attempting to steal a march on its neighbors by rapidly constructing airstrips, ports and other infrastructure on reclaimed land in one of the most sensitive maritime areas in Asia — one with multiple overlapping sovereignty claims. While it probably cannot be stopped, the project should be fully exposed — and China’s attempts to restrict air and sea traffic near its installations decisively rejected.

According to U.S. officials, China has created 2,000 acres of land since 2014 on five coral outcroppings in the Spratly chain, parts of which are also claimed by the Philippines, Vietnam, Brunei, Taiwan and Malaysia. The video released by the U.S. Navy shows dozens of dredging vessels engaged in land-reclamation work, as well as a radar installation and a new airstrip capable of accomodating heavy military aircraft.

Other nations have reclaimed land and built installations in the Spratlys. But China’s operation stands out for its scale and speed and for the brazenness with which the regime of Xi Jinping advances questionable territorial claims while rejecting international mediation or a negotiated code of conduct. Relying on a slapdash map dating to the 1940s, which consists of nine dashes across the waters east of China, Beijing claims 80 percent of the South China Sea, which is crisscrossed by international shipping lanes. It seeks to exclude foreign ships and planes from a 200-mile zone around its claimed territory, rather than the 12 miles recognized by the United States.

The communist regime has dismissed U.S. objections to the island-shaping. “Now, China has the initiative, and as long as China can finish the construction, this round of intervention by the U.S. will end up futile,” crowed the government-run Global Times, according to The Post’s Simon Denyer. Still, overflights — as well as sail-bys by warships that the Pentagon is said to be considering — can make clear that the United States rejects China’s claims. Similar tactics by the United States and Japan neutered a Chinese announcement of an air-defense identification zone in the East China Sea in 2013.

A show of U.S. firmness can also help rally Asian nations that oppose China’s claims and its heavy-handed tactics but that have hesitated to unite against them. As eager as it is to establish its hegemony in the region, China also wants to avoid major conflicts with neighbors and with the United States. It has staged tactical retreats in the past when its maritime aggressiveness has encountered resistance. A regional security summit in Singapore this weekend gives the United States and China’s neighbors an opportunity to push back against the sand castles in the Spratlys. They should join in speaking up.