WASHINGTON—A new Pentagon report says China’s reclamation of landmass among a string of artificial islands in the South China Sea has grown dramatically in recent months, and that Beijing is aggressively patrolling the waters there to assert its territorial claims.

The Pentagon report, issued late Thursday, said that, as of June, China has reclaimed 2,900 acres of landmass across a string of islands in the South China Sea known as the Spratlys, up nearly 50% from May, when the Pentagon said Beijing had claimed about 2,000...