"We stand with our Philippine allies in condemning the PRC’s [the People's Republic of China] efforts to impede lawful operations at Second Thomas Shoal," Ryder said on X (formerly known as Twitter).
The spokesperson added that Washington remains committed to the "ironclad" alliance of the US and the Philippines.
Earlier on Sunday, the coast guard of the Philippines (PSG) "strongly condemned" the Chinese coast guard's "dangerous maneuvers and illegal use of water cannons against the PCG vessels" near Ayungin Shoal, also known as Second Thomas Shoal.
China Coast Guard fired back, saying that two cargo ships and two PSG vessels entered the territorial waters of the Nansha (Spratly) Islands without Chinese permission, adding that China has indisputable sovereignty over the Nansha Islands and its adjacent waters.
In July, PSG accused Chinese coast guard ships of preventing its vessels from approaching the reef.
The Chinese Foreign Ministry rejected the Philippines' accusation, saying that "the Chinese maritime police ships enforced the law in line with regulations" and "acted professionally and proportionately to safeguard China's territorial sovereignty and maritime order."
The reef, known as Ren'ai Jiao in China and Ayungin Shoal in the Philippines, is part of the disputed Spratly Islands, a major archipelago that China and several other Asian nations lay rival claims to. The Philippines grounded a navy transport ship, BRP Sierra Madre, on the reef in 1999 to house a contingent of marines.