Secretary of State Antony Blinken, after speaking with his Philippine counterpart on Wednesday, warned that China’s “actions undermine regional peace and stability” and reiterated the United States’ “ironclad commitments” to the Philippines, according to State Department spokesman Matthew Miller.

The United States shares a mutual defense treaty with the Philippines, and Biden administration officials have stressed in recent months that an armed attack on Philippine military vessels or personnel in the South China Sea would trigger a U.S. military response. The U.S. military is watching the situation at Second Thomas Shoal “very closely,” U.S. Indo-Pacific Command spokesperson Rebecca Rebarich said Wednesday. “We’ve continued to offer assistance to the Filipinos, but there’s not been as of yet any request for help.”

Videos shared by the Philippine military show Chinese coast guard using pickaxes and knives to puncture Philippine rubber dinghies that were attempting to reach the Sierra Madre, and confiscating equipment aboard the navy vessels, including the mobile phones of sailors. The Philippines also said the Chinese blared loud sirens and deployed tear gas to create “chaos” during the confrontation.

A Philippine sailor lost a finger when a Chinese dinghy collided with a Philippine dinghy at high speed, hitting his hand, which was caught between the two vessels, officials said.

At least seven other Philippine sailors were also injured. Chinese ships later towed away two Philippine rubber dinghies after emptying them of their contents.

“This is piracy,” Gen. Romeo Brawner Jr., the Philippine armed forces’ chief of staff, said in a news conference held in Palawan, an island province 120 miles from the reef. “They boarded our boats illegally, they took our equipment. They are like pirates with the actions they carried out.” Brawner added that the Philippine navy personnel “fought back” but were significantly outnumbered by the Chinese and chose not to employ weapons.

“Our objective is that while we want to bring supplies to our troops following international laws, our objective is also to prevent war. We do not want a war,” he said.

At a news briefing Wednesday, Chinese Foreign Ministry spokesman Lin Jian blamed the Philippines for the confrontation and claimed that the Philippine vessels were carrying construction materials and weapons as part of a long-term occupation of the Second Thomas Shoal, which China opposes. “The China Coast Guard took professional and restrained enforcement measures,” Lin Jian said, adding the Chinese forces “did not take direct measures against Philippine personnel.”

Earlier this week, Gan Yu, a spokesman for the Chinese Coast Guard said the Philippine resupply ships “deliberately and dangerously” approached a Chinese ship, resulting in a collision.

China has sought to dominate the South China Sea, a strategic waterway that is also claimed in part by six other governments. Backed by the United States, the Philippines has been ramping up its efforts to push back against the Chinese but is being met with an increasingly forceful response from Beijing that security analysts say could spur broader conflict in the Pacific.

Nowhere is the situation more precarious than at Second Thomas Shoal, said Zack Cooper, a senior fellow at the American Enterprise Institute, where he studies U.S. strategy in Asia. The shoal is one of the only locations where Chinese and Philippine forces are contesting control over physical territory. The Philippines in 1999 beached the Sierra Madre on the shoal and has since populated it with a contingent of marines to create a human presence and stake its claim to the reef.

“It’s the most problematic area in terms of leading to a real, genuine conflict,” Cooper said. What happened on Monday was “very serious,” he added, and would probably lead to calls for the United States to intervene.

Earlier this month, President Ferdinand Marcos Jr. said at a security summit in Singapore that the death of a Filipino citizen through a “willful act” would be “close to an act of war” that could prompt a military response. “Our treaty partner holds that same standard,” he said, referring to the United States.

But the Philippines also has to be wary of when to call upon the United States, said Julio Amador, a Manila-based security analyst. The Philippines has been accused by Beijing of being a “pawn” of the United States, a charge top Philippine officials have rejected. Conducting the resupply mission to Second Thomas Shoal in the face of an imposing Chinese presence is a “an exercise of agency” for the Philippines, Amador said. The government “wants to be able to do this without having to ask for help,” he said.

No Philippine vessels were able to reach the Sierra Madre on Monday to provide supplies for the troops stationed on board, but the navy will try again, Brawner said. “We will defend our rights and we will fight for our future,” he said.

Ship-tracking data showed that as of Wednesday afternoon, dozens of Chinese ships continued to surround Second Thomas Shoal. A 12,000-ton Chinese coast guard vessel, probably the largest of its kind in the world, was also sailing toward to the Philippine island of Thitu near the vicinity of Second Thomas, said Ray Powell, an analyst at Stanford University’s Gordian knot Center for National Security Innovation.

Tan reported from Singapore. Lyric Li in Seoul and Ellen Nakashima in Washington contributed to this report.