Ashton Carter's stop there is likely to further rile Beijing, which claims much of the sea as its own and is building manmade islands there.
US Defense Secretary Ash Carter says he will be visiting an aircraft carrier in the South China Sea, the visit coming a day after he announced new military aid to the Philippines that spurred protests from China.
It would be the second time that Carter has flown onto a carrier while it is embarked in the hotly contested South China Sea. He will be on the USS John C Stennis, but it is not clear exactly where it is afloat in the South China Sea.
Carter's stop there is likely to further rile Beijing, which claims much of the sea as its own and is building manmade islands there. China is stationing aircraft and other weapons systems on the islands.
Speaking at a closing ceremony for a joint US-Philippines military exercise, Carter said the US "will continue to stand up for our safety and freedoms" and those of America's friends and allies. And he said the US is committed to ensuring that the Asia Pacific remains a region where every can rise and prosper.
Last November he was on the USS Theodore Roosevelt as it sailed northwest of Borneo.
During that visit, Carter acknowledged his stop was being noted because of the tensions in the region over the island disputes.
"There's a lot of concern about Chinese behavior out here," he said.
A week earlier, a US Navy destroyer, the USS Lassen, challenged China's claim to a 12-mile territorial limit around Subi Reef in the Spratly Islands about 150 miles to 200 miles from where the Theodore Roosevelt was sailing.