Rubio's remarks came after the Philippines accused a Chinese coast guard vessel of firing a water cannon at a government fisheries vessel near Scarborough Shoal on Thursday, with the blast scoring an indirect hit.
"The whole world saw it, and it's disturbing," Rubio told reporters on the sidelines of the ASEAN meeting in Manila.
"We have a role, and we're not going to abandon our allies. We've made that abundantly clear."
Thursday's incident followed a confrontation on Monday at the Second Thomas Shoal, where the Philippine Military said a Filipino sailor was injured after being struck with a baton by Chinese Coast Guard personnel.
The US State Department called China's actions "dangerous and provocative".
The Scarborough Shoal, like the Second Thomas Shoal, has become a flashpoint in the already tense relationship between the Philippines and China, which remain locked in a territorial dispute despite a 2016 arbitral ruling that found China's claims in the South China Sea had no legal basis. China rejects that ruling.
A violent confrontation at the Second Thomas Shoal in June 2024, where a Filipino sailor lost a finger, prompted the Philippines and China to reach a provisional arrangement on resupply missions.
Manila's foreign ministry said on Thursday that the arrangement neither recognises any foreign jurisdiction nor requires the Philippines to ask China for permission to carry out resupply missions.
The Philippines regularly sends supplies to a small number of soldiers aboard a World War Two-era warship BRP Sierra Madre, which it grounded on Second Thomas Shoal in 1999 to assert its claims.
China on Thursday said it tracked and drove away two Philippine vessels it accused of having "intruded into" waters near the disputed Scarborough Shoal, which it calls Huangyan Dao, despite a warning.
"We urge the Philippines to immediately cease its infringing and provocative actions," China's Coast Guard said in a statement, while reaffirming China's sovereignty claims over the disputed shoal.
The Coast Guard took necessary measures, such as tracking, monitoring, and interception to drive away the Philippine vessels, it said, calling the actions "standard, professional and lawful".
