The U.S. Navy is casting a wary eye on a disputed atoll less than 150 miles from the Philippines' capital amid concerns the Chinese are preparing to convert it into their latest island base.

Philippine president Rodrigo Duterte summoned the Chinese ambassador Monday to explain a flotilla of ships near Scarborough Shoal, a flashpoint in the protracted conflict between China and its neighbors in the South China Sea. The U.S. Navy, which has announced joint patrols with the Philippines, is weighing whether to patrol near the disputed reef, according to U.S. officials who spoke to Navy Times.

Officials say there is no solid evidence China is ready to turn Scarborough Shoal into their latest man-made island, but the concerns are putting military leaders on edge. Philippine officials claimed dredgers and barges were seen, according to a Wall Street Journal report. An island building project so close to the Philippines mainland would put U.S. forces at considerable risk in the event of a conflict, according to defense experts.

Two U.S. officials who asked for anonymity to discuss intelligence assessments said it was far from clear that China was launching another island building project such as others it has constructed in the Spratly Islands. Images circulating of the flotilla do not seem to show any dredgers or command and control barges that have been used in past projects.  

The officials say there does appear to be stepped up Chinese activity in the reef, which is well within the Philippines’ exclusive economic zone and that China seized in 2012.

"The worry is that this kind of stepped up activity has been a precursor to island building in the past," said one U.S. official.

Hua Chunying, a spokeswoman for China’s Foreign Ministry, on Monday denied that China was starting land reclamation there, and said the situation was unchanged, according to The Washington Post.

There does not seem to be any evidence that construction has begun on Scarborough Shoal, said a China expert at the Center for Strategic and International Studies who believes the uptick in activity -- which includes Chinese coast guard and likely some paramilitary maritime militia -- is probably a message to the Philippines.

"It would be consistent with the approach China has been taking to dealing with other countries," said Bonnie Glaser; who directs the China Power Project at CSIS. "It’s likely a move to put pressure on the Philippines ahead of discussions on the bilateral relationship. A combination of carrots and sticks is generally how [Chinese President] Xi Jinping deals with other countries."

Shifting 'status quo'

Scarborough Shoal is the latest and possibly most dangerous flash-point over the simmering South China Sea, due to the reef's nearness to a U.S. ally who has agreed to allow base ​more rotating of U.S. forces. Building here would also demonstrates​ that U.S. and other neighbors have failed to cow China's most provocative moves. 

"When it comes to the South China Sea, I think the largest military concern for [U.S.] Pacific Command is what operational situation will be left to the next commander or the commander after that," one defense expert, who'd been briefed on South China Sea developments by military leaders, told Navy Times in April. "The status quo is clearly being changed. Militarization at Scarborough Shoal would give [China's People's Liberation Army-Navy] the ability to hold Subic Bay, Manila Bay, and the Luzon Strait at risk with coastal defense cruise missiles or track aviation assets moving in or out of the northern Philippines."

During an April trip to the region, Defense Secretary Ash Carter announced more U.S. forces would be rotating through the Philippines under a recently signed agreement.

The U.S. Navy has been regularly patrolling the waters near Scarborough Shoal and other contested features in the run-up to and since a landmark ruling by an international tribunal that invalidated China’s claim to economic rights to most of the South China Sea. China rejected the court’s ruling, claiming it did not have jurisdiction over the matter.

Another U.S. patrol near Scarborough Shoal is likely in the coming days, officials say. This sort of patrol demonstrates the U.S. position that these are international waters and could also support a surveillance assessment. 

The Navy has been regularly operating destroyers in the South China Sea, including a three-ship surface action group deployed there. Navy Times reported in July that destroyers had been regularly stalking China’s claims but often outside the 12-mile territorial limit that China claims.


The distance is important because if the ships patrolled within 12 miles, the Navy would handle it as a freedom of navigation operation that asserts U.S. rights to freely transit in waters claimed by other countries.

Those FONOPS patrols must be approved at very high levels, unlike close patrols outside the 12-mile boundary. Experts say the tactic sends a message of resolve to China. to the Chinese and U.S. allies in the region.

A U.S. Pacific Fleet spokesman deferred comment to the State Department, but confirmed that the destroyer Spruance is operating in the South China Sea.

Spruance, along with the Momsen and Decatur, make up the three-ship surface action group.