WASHINGTON—The United States vowed on Thursday to keep up air and sea patrols in international waters even after the Chinese Navy warned a US surveillance plane eight times to leave the airspace over artificial islands it is building in the disputed South China Sea.

READ: ‘Go away,’ China tells US spy plane in West PH Sea — report

The Pentagon also said US surveillance aircraft and naval ships have yet to test China’s territorial claims around artificial islands, but that could be “the next step.”

Although the United States does not recognize China’s claims of sovereignty around the man-made structures, American P-8 surveillance planes and naval vessels patrolling the area have not ventured within 22 kilometers of the artificial islands—the standard territorial zone around natural land.

“That would be the next step,” Col. Steven Warren, spokesperson for the Pentagon, told reporters.

Asked if the military would move to within that sensitive zone, he said: “We don’t have any announcement to make on next steps. We are going to continue our routine flights.”

The senior US diplomat for East Asia, Assistant Secretary of State Daniel Russel, told a media briefing in Washington that Wednesday’s US reconnaissance flight was “entirely appropriate” and that US naval forces and military aircraft would “continue to fully exercise” the right to operate in international waters and airspace.

He said the United States would go further to preserve the ability of all countries to move in international waters and airspace.