BEIJING—In a broadening campaign to enforce its territorial claims, China says it’s beefing up its police powers in the disputed South China Sea, presumably including waters known in Manila as West Philippine Sea, and requiring foreign fishermen to ask Beijing’s permission to operate within most of the vast, strategic waterway.

The move, which took effect this month, comes on the heels of the late November announcement of a new air defense zone requiring foreign planes to notify Beijing of flights over a huge swath of the East China Sea, where China is locked in a bitter territorial dispute with Japan.

The steps are prompting concerns that President Xi Jinping’s push to assert China’s role as a regional power could spark a confrontation with neighbors.

“These sort of assertions of sovereignty, or territorial claims, will continue. Xi believes he can’t afford to be seen as soft,” said Joseph Cheng, a politics expert at City University of Hong Kong China.