MANILA (Reuters) - China sent several ships to a disputed atoll in the South China Sea, preventing Filipino fishermen from accessing traditional fishing grounds and raising tensions in the volatile region, Philippine officials said on Wednesday.

Soldiers of China's People's Liberation Army (PLA) Navy patrol at Woody Island, in the Paracel Archipelago, which is known in China as the Xisha Islands, January 29, 2016. REUTERS/Stringer

China had sent as many as seven ships to Quirino Atoll, also known as Jackson Atoll, in recent weeks, said Eugenio Bito-onon Jr, the mayor of nearby Pagasa Island in the Spratly Islands.

The Spratlys are the most contested archipelago in the South China Sea, a resource-rich region and critical shipping lane linking North Asia to Europe, South Asia and the Middle East.

“This is very alarming, Quirino is on our path when we travel from Palawan to Pagasa. It is halfway and we normally stop there to rest,” Bito-onon told Reuters.

“I feel something different. The Chinese are trying to choke us by putting an imaginary checkpoint there. It is a clear violation of our right to travel, impeding freedom of navigation,” he said.

In Beijing, Foreign Ministry spokesman Hong Lei said China’s Ministry of Transport had sent vessels to tow a grounded foreign ship and they had since left the surrounding waters.

“To guarantee safety of navigation and of work conditions, China urged fishing vessels near the site to leave,” Hong said, adding that China had indisputable sovereignty over the atoll.

The Philippines Foreign Ministry said Chinese coast guard vessels had been seen at the atoll two weeks ago but were not in the area on Wednesday.

“The Department is monitoring reports on the situation on the ground and reiterates its call for China to exercise self-restraint from the conduct of activities that could complicate or escalate disputes in the South China Sea and affect peace and stability in the region,” the ministry said in a statement.

TENSIONS ON THE RISE

Earlier, the Philippine military said it was looking into the situation around Jackson Atoll, where a Chinese warship allegedly fired warning shots at Filipino fishermen in 2011.

“We know there are Chinese ships moving around the Spratly area,” spokesman Brigadier-General Restituto Padilla told Reuters. “There are also ships around Second Thomas Shoal, so we want to make sure if the presence is permanent.”

A spokesman for the U.S. State Department said it was trying to confirm the latest reported incident.

Mark Toner told a regular news briefing that the United States, a treaty ally of the Philippines that has repeatedly expressed concerns about Beijing’s methods in pursuit of maritime claims, did not want to China using its ships “to intimidate ... fishing vessels in that region.”

Second Thomas Shoal is where the Philippine navy has been occupying and reinforcing a rusting ship it ran aground in 1999 to bolster its claims to the disputed reef.

A military source from Palawan said a surveillance plane had seen four to five ships in the vicinity of Jackson Atoll last week.

“There are no indications China will build structures or develop it into an island,” said the source, who was not authorized to speak to the media about the South China Sea.

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The Philippines Star newspaper, which earlier reported the story, quoted an unidentified fisherman as saying Chinese boats chased them away when they tried to enter the area last week.

Along with China and the Philippines, Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims on the waters, through which about $5 trillion in trade is shipped every year.

Tensions have been building recently, with the United States and others expressing concerns about China’s land reclamation in the Spratly Islands and deployment of surface-to-air missiles and fighter jets in the Paracel Islands.

U.S. Defense Secretary Ash Carter warned China on Tuesday against what he called “aggressive” actions in the region, saying there would be “specific consequences” to militarization of the South China Sea.

In response, Hong urged Washington on Wednesday to “stop exaggerating and sensationalizing” the issue.

For its part, Beijing has been angered by “freedom of navigation” air and sea patrols the United States has conducted near the islands it claims in the South China Sea and says it needs military facilities for its self defense.