Philippine authorities shows Chinese vessels anchored at Whitsun Reef in March 2021 amid rising tensions
"The United States stands with our ally, the Philippines, in the face of this escalation that directly threatens regional peace and stability," State Department spokesman Ned Price said in Abuja, where Secretary of State Antony Blinken was travelling.
The action "escalates regional tensions, infringes upon freedom of navigation in the South China Sea as guaranteed under international law and undermines the rules-based international order."
He warned that any "armed attack on Philippine public vessels" would invoke the 1951 US-Philippines treaty in which Washington is obliged to defend its ally.
The Philippines said the Chinese Coast Guard on Tuesday fired a water cannon against boats delivering supplies to Filipino marines, forcing them to halt their mission.
The incident came as the Filipinos were travelling to Second Thomas Shoal in the Spratly Islands, one of a series of hotly contested areas in the South China sea.
The Philippines earlier voiced "outrage, condemnation and protest".
China defended its actions, saying that it acted to "safeguard China's sovereignty" as the Filipinos had not been in contact on their movements.