The United States vowed on Thursday to keep up air and sea patrols in international waters after the Chinese navy repeatedly warned a U.S. surveillance plane to leave the airspace over artificial islands China is creating in the disputed West Philippine Sea.

The incident happened on Wednesday when a Chinese navy repeatedly warned a U.S. P8-A Poseidon, America’s most advanced surveillance and submarine-hunting aircraft to leave airspace around disputed islands in the West Philippine 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, were reported by a CNN team aboard the plane.

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

Video Footage courtesy of CNN.

Wednesday’s mission was specifically aimed at monitoring Chinese activities on three islands that months ago were reefs barely peaking above the waves. Now they are massive construction projects that the U.S. fears will soon be fully functioning military installations.

China claims most of the potentially energy rich South China Sea, through which $5 trillion in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Brunei and Taiwan also have overlapping claims.

reclamation of china in spratly
Still image from United States Navy video shows a U.S. Navy crewman aboard a surveillance aircraft viewing a computer screen purportedly showing Chinese construction on the reclaimed land of Fiery Cross Reef in the disputed Spratly Islands

The United States will continue to patrol, from air and sea, the area around the Spratly islands, Army Col. Steve Warren said. International law does not recognize man-made islands as extensions of the mainland, Warren said.

The Navy and Air Force patrols are conducted to ensure freedom of navigation, Warren added.