China’s coast guard ships have swarmed and collided with Philippine boats. They have doused Philippine vessels with powerful water cannons. Chinese crew members have slashed inflatable crafts, blared sirens and flashed high-powered lasers at Filipino troops.

As China pushes to dominate the South China Sea, it is increasingly willing to use force to drive out the Philippines, a treaty ally of the United States. In recent months, China’s tactics have damaged Philippine boats and injured personnel, and raised fears of a superpower showdown in the strategic waterway.

A New Flashpoint

For months, the latest target of China’s power play was a Philippine coast guard ship, the Teresa Magbanua. The video above was taken by the crew of that ship, as a Chinese coast guard vessel collided into it late last month.

The episode was one of four confrontations between the two countries’ vessels, in just two weeks. The encounters were not only becoming more frequent, but they were also taking place in a new location — Sabina Shoal, a resource-rich atoll close to the Philippine mainland.

The two countries had in earlier months been facing off near another atoll in the disputed Spratly Islands, the Second Thomas Shoal, where Chinese ships regularly harass Philippine boats trying to resupply sailors stationed on a beached warship. Now, their feud has expanded.

The Philippines wants to control Sabina Shoal, an unoccupied atoll inside its exclusive economic zone. Sabina Shoal, which lies just 86 miles west of the Philippine province of Palawan and over 600 miles from China, is near an area rich in oil deposits, and on routes Manila considers crucial for trade and security.

“A hostile China would be able to strangle our maritime trade with the rest of Asia and most of the world from Sabina Shoal,” said Jay Batongbacal, a maritime security expert at the University of the Philippines. Sabina Shoal would make “a good staging ground for vessels that will interfere with Philippine maritime activities,” he said.

Manila anchored the Teresa Magbanua, one of its largest coast guard ships, at the Sabina Shoal in April to try to stop China from what the Philippines sees as efforts to try to build an island there.

The Philippine Coast Guard has pointed to piles of crushed and dead corals apparently dumped on the shoal as signs of Chinese land reclamation under way. China has denied the accusation. But the building and fortifying of artificial islands is a key part of how China has asserted its claims over contested waters hundreds of miles from its coast.

China, which claims almost all of the South China Sea, says its tactics are needed to defend its sovereignty. Beijing has rejected a ruling by an international tribunal in 2016 that China’s sweeping claim to the waters had no legal basis.

China accused the Philippines of trying to permanently occupy Sabina Shoal by parking the coast guard vessel on it, just as it had grounded the warship at Second Thomas Shoal. Beijing even sent tugboats to Sabina Shoal, which some read as a threat to tow the Philippine ship away.

China has not resorted to guns. Rather, it is using what military theorists call gray zone tactics, aggressive moves that fall short of inciting all-out war. That includes imposing blockades, blasting water cannons and sailing dangerously close.

But the moves can still cause damage: The recent collision between Chinese and Philippine boats, for instance, left a three-foot hole on the Teresa Magbanua, as well as another Philippine vessel.

“If the Philippines insists on occupying more shoals, China will have no choice but to use all available measures,” said Hu Bo, director of the South China Sea Strategic Situation Probing Initiative, a Beijing-based research group. “There is no limit.”

On Sunday, after months of pressure from China, the Philippines said that the Teresa Magbanua had returned to port in Palawan. The Philippine statement sought to cast the move as following the accomplishment of the boat’s mission.

But it nodded to the challenges of remaining in the face of a Chinese blockade that prevented the ship from being resupplied, saying the crew had been “surviving on diminished daily provisions” and that some needed medical care.

The Philippines said the vessel had suffered structural damage from being rammed by the Chinese coast guard, but indicated that the boat would return after undergoing repairs.

Tensions on the Rise

President Ferdinand R. Marcos Jr. of the Philippines has taken on a more muscular approach against China than his predecessor did. He has beefed up the country’s alliance with the United States and invited journalists to join resupply missions at sea to highlight China’s actions.

China has called the United States “the biggest troublemaker stirring up unrest in the South China Sea.” Mr. Hu, the expert in Beijing, said that China has been compelled to use heavier-handed tactics because diplomacy with the Marcos administration has failed.

With both sides digging in, they are tangling with each other more often and more aggressively.

In one confrontation in June, China’s coast guard used axes, tear gas and knives to harass Philippine troops on a resupply mission to the Second Thomas Shoal. Chinese sailors punctured Philippine military boats and seized their equipment, including guns.

Eight Filipino soldiers were hurt, including one who lost a finger. The Philippine military called it the “most aggressive” Chinese action in recent history.

That episode on June 17 made clear that tensions needed to be dialed down. The two sides briefly came to a “provisional agreement” on the Second Thomas Shoal, and the Philippines was able to conduct a resupply mission at the end of July. But officials from both countries have disputed the details of the agreement, raising questions about how long it will last.

“China’s overarching strategy is to dominate the South China Sea. We should not expect the de-escalation to last,” said Rommel Ong, a professor at the Ateneo School of Government in Manila and a retired rear admiral in the Philippine Navy. “Unless they attain that objective, their coercive actions will wax and wane depending on the situation.”

Since October, the Chinese coast guard has used water cannons against Philippine ships more regularly than it likely ever has in the long-running dispute. Collisions have also become more common.

Whenever the Philippines has attempted to sail to disputed atolls, ships from the Chinese coast guard, maritime militia, and navy have rapidly confronted them.

Some of the Chinese ships shadow the Philippine boats. Others cut across their paths. The ships swarm around the Philippine vessels to form a tight blockade.

China, which boasts the world’s largest navy in terms of the number of vessels, has been deploying more boats to these disputed waters over the past year than it did previously. The Philippines sends on average a few ships on its resupply missions, which has mostly remained unchanged.

Mr. Hu, the Chinese expert, said that China’s show of strength in numbers is meant to deter the Philippines without resorting to lethal force. “If China sends only a small number of boats to stop the Philippines, they might have to use guns,” he said.

From Aug. 27 to Sept. 2, a weeklong period, the Philippine military tracked 203 Chinese ships in contested areas in the South China Sea — the highest number recorded this year.

Tensions have risen at a time when the militaries of China and the United States have had limited contact. On Tuesday, the commander of U.S. Indo-Pacific Command held a rare video conference with Gen. Wu Yanan, the commander of the People’s Liberation Army’s Southern Theater Command, which oversees the South China Sea. The United States said such calls help “reduce the risk of misperception or miscalculation.”

During the call, Adm. Samuel Paparo urged China to “reconsider its use of dangerous, coercive, and potentially escalatory tactics” in the South China Sea. China, in its own statement about the call, said only that the two sides had an in-depth exchange of views.

On Thursday, though, Lieutenant General He Lei, a former vice president of the People’s Liberation Army’s Academy of Military Sciences, struck a more hawkish note.

“If the United States insists on being a plotter that pushes others to stand on the front line to confront China, or if it has no other choice but to challenge us by itself,” he told reporters at a security forum in Beijing, “the Chinese people and the People’s Liberation Army will never waver.”