China’s plans to build up a disputed island near the Philippines could lead to a regional conflict, Defense Secretary Ash Carter told Congress on Thursday.

Carter was asked about the strategic significance of China’s plan to add military facilities to a disputed island known as Scarborough Shoal located about 120 miles—within missile range—of Subic Bay, Philippines, where U.S. warships will be based.

The defense secretary said Scarborough is "a piece of disputed territory that, like other disputes in that region, has the potential to lead to military conflict."

"That's particularly concerning to us, given its proximity to the Philippines," Carter told a Senate Armed Services Committee hearing on efforts to counter the Islamic State terrorist group.

Carter said the Pentagon has similar views about a number of island disputes in the South China Sea, where China has been claiming most of the sea as its maritime territory and demanding that other states, including the United States, keep out of the region.

The comments came in response to questions posed by Sen. Dan Sullivan (R., Alaska) who revealed during the hearing that Scarborough is the third vertex of a triangle of Chinese military bases that could be used to threaten the main Philippines island of Luzon.

A map showed Woody Island in the Paracels and three other islands in the Spratlys Islands that are undergoing militarization by the Chinese as part of the island building program.

"The Chinese have already established two legs of that triangle. The fighters and radars are part of that radius that you see around the Scarborough Shoal," he said.

Carter said the map was "absolutely accurate" in depicting a Chinese strategy for creating an island triangle of bases in the sea.

Later, Sullivan told the Washington Free Beacon he is "very concerned" by reports that China will make Scarborough the "third site" for missiles and warplanes close to Subic Bay.

"In addition to seizing and building on a shoal long claimed by the Philippines, a militarized Scarborough—with an air-search radar—would give the PRC full overwatch of flights in and out of northern Philippines, and the deployment of coastal defense cruise missiles there would allow the PRC to hold U.S. forces based and operating in the Philippines at risk," Sullivan said, using the acronym for People’s Republic of China. "The strategic implications for U.S. and allied forces operating in Southeast Asia are undeniable."

Earlier, Sen. Tom Cotton (R., Ark.) noted that the Chinese military could monitor all flights from the northern Philippines using radar and could "hold Subic Bay, Luzon Strait, and Manila Bay at risk with missile systems on Scarborough Shoal."

Carter said concerns about the shoal are a reason for closer defense ties with Manila. "They're a treaty ally. We take that seriously, very seriously. That's why we are establishing some new installations from which we can operate, so that we strengthen our own posture there," he said.

The Pentagon is sending warships and aircraft to the Philippines as part of what is called an enhanced defense agreement.

Tensions have been heightened by Chinese military activities in the sea. A legal showdown is expected in the next several weeks when an international court is expected to rule in favor of the Philippines and against China’s expansive South China Sea claims.

In Beijing, a Chinese Defense Ministry spokesman bristled at U.S. opposition to the island building campaign and accused Washington of heightening tensions.

Col. Wu Qian, the spokesman, also rejected U.S. concerns about a pending military buildup on Scarborough, called Huangyan Island by the Chinese.

Dismissing reports of the buildup as "media hype," Wu said Scarborough "is an inherent territory of China and China has the right to take measures to deal with various kinds of threats and safeguard the sovereignty and security of China."

He declined to answer when asked if military forces are being deployed on the shoal.

The Pacific Air Forces, a command of the U.S. Air Force, carried out three A-10 jet missions near Scarborough on April 19, 21, and Tuesday. The jets were deployed at Clark Air Base in the Philippines.

Carter came under harsh criticism from Committee Chairman Sen. John McCain (R., Ariz.) after the defense secretary said details about the recent flights are being kept secret.

"This is the second time, Secretary Carter, that you've refused to confirm what is well known in the media. That's not fair to this committee. It's all been reported there were flights into the area around those islands," McCain said.

"But to classify the fact that we are sending our ships and airplanes into international waters and have that classified, when it should be magnified throughout the world that United States is asserting our respect and adherence to international law, is something that is confusing and befuddling," McCain said.

"Why would we want to classify the fact that we are doing what every nation in the world should be able to do? And that's sail or fly wherever we want to. Why should that be classified information?"

In an earlier hearing, Carter refused to discuss a Navy warship "freedom of navigation" operation until pressed to confirm the warship passage by McCain.

Carter said he would look into why the South China Sea naval and aircraft activities are being kept secret. He said he was trying to be "respectful" to secrecy rules by not commenting.

The secretary appeared unaware that the Hawaii-based Pacific Air Forces issued a press release last week about the April 19 overflight mission near Scarborough by four A-10s and two HH-60 helicopters.

"Our job is to ensure air and sea domains remain open in accordance with international law. That is extremely important, international economics depends on it—free trade depends on our ability to move goods," Col. Larry Card, commander of the Pacific Air Forces contingent in the Philippines, was quoted as saying.

Carter said the nations of the region and surrounding areas have expressed concern about Chinese military activities in the sea and "that’s why we’re doing the rebalance" of U.S. forces to Asia.

The Chinese activities have led to strengthened alliances and the creation of new partnerships with states such as Vietnam and India, he said.

The Chinese buildup is "also why we're sending our best equipment to the Asia Pacific, why we're doing more," Carter noted.

Cotton said reports of the three U.S. overflights indicated the jets did not pass within 12 miles of the shoal and as a result diminished the effort to contest Chinese territorial claims to the shoal.

Retired Navy Capt. Jim Fanell, a former Pacific Fleet intelligence chief, said China appears to be seeking control of key chokepoints of the Luzon Strait by building up Scarborough Shoal.

"When combined with the seven new islands at the southern end of the Spratly Islands, a new naval base at the northern entrance will provide the PRC the ability to effectively control the freedom of navigation and free access to markets for all nations who ply the waters of the South China Sea," Fanell said.

To counter China’s moves, Fanell said the Navy’s Pacific Fleet should begin warship patrols at Scarborough.

"This ‘Scarborough Patrol Station’ should be tasked with preventing the PRC from physically building a ‘new island,’ presuming afloat commanders are given flexible rules of engagement to allow them to physically prevent PRC construction work," he said, adding allies such as Japan, Australia, and India should be part.

"To take no action is to cede the entirety of the South China Sea to Beijing and their concepts of ‘freedom of navigation’ with socialist characteristics," he said. "Time is short and action, even in this presidential election year, is required now."

Regarding U.S. military operations against the Islamic State, Carter said progress is being made and that targeted strikes have killed "ISIL’s cabinet," including its ministers of war and finance.

"And our attacks on ISIL's economic infrastructure from oil wells and trucks to cache storage to ISIL's financial leaders is putting a stranglehold on ISIL's ability to pay its fighters, undermining its ability to govern and making it harder to attract new recruits," Carter said.

Marine Corps Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, offered a more cautious assessment but asserted that efforts to destroy ISIL are making real progress.

"That said, we're not satisfied or complacent about where we are, and we won't be satisfied until ISIL is defeated in Iraq and Syria and wherever it attempts to take root," said Dunford, who testified with Carter.

Carter said the number of foreign fighters joining ISIS also has decreased.

Dunford said the decline in foreign fighters joining ISIS was due to increased efforts by the originating countries and greater efforts by Turkey to stem the flow into Syria and Iraq.

A recent poll indicated Islamic youth are less attracted to the terror group than they were last year.

"The less success they have on the battlefield, the less an appeal there is; the less of the appeal they have to be a global caliphate," Dunford said.

Dunford also disclosed that the U.S. military is capable of destroying the Syrian government air force that has caused most of the estimated 300,000 deaths in the Syrian civil war.

Asked why the air force has not been targeted, Dunford said: "We have not declared war on the Syrian regime, senator."

Asked by Sen. Roger Wicker (R., Miss.) if a congressional declaration of war were needed to attack the Syrian air force, Dunford said: "I think it would take the president directing us to do that, senator … The task [the president] has given us militarily is against ISIL, senator."