The U.S. military is considering using aircraft and Navy ships to directly contest Chinese territorial claims to a chain of rapidly expanding artificial islands, U.S. officials said, in a move that would raise the stakes in a regional showdown over who controls disputed waters in the South China Sea.

Defense Secretary Ash Carter has asked his staff to look at options that include flying Navy surveillance aircraft over the islands and sending U.S. naval ships to within 12 nautical miles of reefs that have been built up and claimed by the Chinese in an area known as the Spratly Islands.

Such moves, if approved by the White House, would be designed to send a message to Beijing that the U.S. won’t accede to Chinese territorial claims to the man-made islands in what the U.S. considers to be international waters and airspace.

The Pentagon’s calculation may be that the military planning, and any possible deployments, would increase pressure on the Chinese to make concessions over the artificial islands. But Beijing also could double down, expanding construction in defiance of the U.S. and potentially taking steps to further Chinese claims in the area.

The U.S. has said it doesn’t recognize the man-made islands as sovereign Chinese territory. Nonetheless, military officials said, the Navy has so far not sent military aircraft or ships within 12 nautical miles of the reclaimed reefs to avoid escalating tensions.