BEIJING – The Chinese navy repeatedly warned a U.S. surveillance plane to leave airspace around disputed islands in the South China Sea – a sign that Beijing may seek to create a military exclusion zone in a move that could heighten regional tensions.

The warnings, delivered eight times to a P-8A Poseidon over the Spratly Islands on Wednesday, were reported by a CNN team aboard the plane.

“Foreign military aircraft. This is Chinese navy. You are approaching our military alert zone. Leave immediately,” a radio operator told the aircraft, later bluntly warning: “Go, go.”

After each warning, the U.S. pilots responded calmly that the P-8A was flying through international airspace, according to the CNN team.

China claims sovereignty over more than 80 percent of the South China Sea. Rival claimants to islands and reefs – set amid fertile fishing grounds and potentially oil- and gas-rich waters – include the Philippines, Vietnam, Taiwan, Malaysia and Brunei.

In the Spratly Islands, China has been engaged in a massive program of land reclamation and construction, including building artificial islands.

On Thursday, a spokesman for China’s Foreign Ministry, Hong Lei, said Beijing “has the right to monitor certain airspace and maritime areas and safeguard national security, to prevent unexpected incidents at sea.”

He added that other countries should respect China’s sovereignty.

An attempt to impose restrictions in what is widely seen as international airspace would significantly raise tensions in the area and provoke confrontations between the U.S. and Chinese militaries, experts said.

Images captured by the U.S. plane’s high-performance cameras showed dozens of dredging vessels at different islands, some pumping sand to build new land out of the ocean.

They also showed an early-warning-radar building and a new airstrip on Fiery Cross Reef that CNN reported was long enough to land any military aircraft operated by China.

Capt. Mike Parker, on board the aircraft, said he thinks that at least one of the verbal challenges came from the radar station.

“Although China glosses over the military purpose of those artificial islands, they are likely primarily intended to change the power balance in the South China Sea vis-a-vis the U.S. Navy, which for now is the dominant force in the area,” said Yanmei Xie, senior China analyst at the International Crisis Group.

China could use the completed installations to “scramble fighter jets to intercept, tail and attempt to evict incoming military aircraft,” Xie noted.

“That scenario would turn the South China Sea into a theater of frequent near-misses and even clashes,” she said.

Under international law, the construction of artificial islands confers no right of sovereignty over neighboring waters, and the United States has made it clear that it will not respect China’s claim to what it sees as international waters and airspace.

In Washington, a Pentagon spokesman, Col. Steve Warren, said “freedom of navigations operations” would continue in the South China Sea, but he insisted that U.S. military aircraft do not fly directly over areas claimed by China in the Spratly Islands.

“We will continue to fly in international airspace,” he said.

While Secretary of State John Kerry expressed concern about China’s land reclamation project to the nation’s leaders last weekend, his complaints appeared to fall on deaf ears.

Foreign Minister Wang Yi said China’s determination “to safeguard our own sovereignty and territorial is as firm as rock and is unshakable.”

But on social media, some Chinese mocked the failure to scare off the U.S. plane.

“Isn’t intercepting the robbers in the air the responsibility of the Chinese air force?” one asked. Another branded the incident a “national disgrace and a disgrace for the Chinese people.”

Although China has acknowledged that the islands will have military uses, Hong insisted that the main purpose of the construction work was “to provide service for search and rescue at sea, fishing security, disaster prevention and relief, and meteorological monitoring, among other things.”

Last week, senators on both sides of the aisle in Washington called for a more robust U.S. response to China’s maritime activity, arguing that China was not paying any price for its actions while regional allies were questioning U.S. commitment to Asian security.

Paul Haenle, director of the Carnegie-Tsinghua Center in Beijing, said Chinese President Xi Jinping has taken a much more assertive approach to strengthening China’s maritime claims.

“After several decades of being weak, the Chinese feel they have lost ground on their historical claims and are now in a better position to strengthen them. And the lack of strong U.S. leadership internationally has contributed to a sense in China that they can push these claims now and will not face negative consequences,” he said.

Deputy Secretary of State Antony Blinken said at a conference in Jakarta, Indonesia, on Wednesday that China’s actions were eroding regional trust and could provoke conflict.

“Its behavior threatens to set a new precedent whereby larger countries are free to intimidate smaller ones, and that provokes tensions, instability and can even lead to conflict,” he said, according to the Reuters news agency.

But in a sign that the U.S. and Chinese militaries have taken measures to improve communication and avoid clashes, a U.S. combat ship used agreed codes for unplanned encounters when it met a Chinese vessel during a recent patrol of the South China Sea.

“We exchanged messages, and it was very professional,” Cmdr. Matthew Kawas, the commanding officer of the USS Fort Worth, told visiting journalists in Singapore on Wednesday.

He declined to comment further on the communications with the Chinese vessel, other than to point out that it is useful for both navies to become accustomed to each other’s practices.

Earlier, Adm. Michelle Howard told reporters in Singapore that the two navies had agreed to use codes specifically designed to manage unplanned encounters at sea.

“Fort Worth came across one of our counterparts and they did do that, so things went as professionally as they have since that agreement was made,” she said, according to Bloomberg News.

If the Navy acts on the proposal to step up patrols in the South China Sea, the Fort Worth, a littoral combat vessel, and its sister ships are likely to play a key role.

The expensive new additions to the Navy’s fleet are speedy and maneuverable and have a draft of just 15 feet.

“It enables us to go places where other ships cannot,” said Capt. Fred Kacher, commodore of the Navy’s Destroyer Squadron 7, adding that an unmanned helicopter on board the ship is equipped with a a video camera that allows the Fort Worth “to see what’s going on.”