Chinese president Xi Jinping has reportedly ordered the People's Liberation Army to prepare for combat.
This comes ahead of an international tribunal on Tuesday that's expected to issue an unfavorable ruling against China's claims over the South China Sea.
U.S.-based Boxun News said Tuesday that the instruction was given in case the United States takes provocative action in the waters once the ruling is made.
The U.S. and China have been expanding their military activities across the sea, stoking heavy tension between the two superpowers.
China controls roughly 90 percent of the South China Sea, a critical waterway that handles some five trillion U.S. dollars worth of trade every year.
The Philippines filed a suit to The Hague tribunal in 2013, accusing Beijing of seizing Scarborough Shoal, a fishing area that Manila claims is part of its exclusive economic zone.
It said China's extensive maritime claims don't conform to the UN Convention on the Law of the Sea which was ratified by both countries.
Since then, the country has stacked up 15 claims against China's vast territorial expansion.
China opposes the arbitration process.
It says it does not acknowledge the tribunal's jurisdiction, and will not accept its ruling.
The Hague's ruling on Tuesday is binding but the court lacks powers of enforcement.
Oh Soo-young, Arirang News.