Vietnam said Chinese boats yesterday intentionally rammed Vietnamese vessels during a confrontation over the placement of an exploration rig by China in waters near the Paracel Islands, claimed by both countries. Armed Philippine police also arrested Chinese fishermen near a disputed shoal close to the disputed Spratly Islands.

China responded by accusing both countries of violating its sovereignty over the island chains, saying Vietnam was being “disruptive” and demanding the release of the boat crew by the Philippines.

The incidents come as China’s Asian neighbors have been seeking closer ties with the U.S. as they push China to agree to a code of conduct to avoid conflicts over seas that are rich in oil and gas. Chinese President Xi Jinping is expanding the country’s naval reach to back its claims to huge swaths of the South China Sea that is based on China’s so-called “nine-dash line” map, first published in 1947, that extends hundreds of miles south from China’s Hainan Island to the equatorial waters off the coast of Borneo.

“The risk of escalation is real, given the role of oil and proximity to both countries,” said Taylor Fravel, a professor at the Massachusetts Institute of Technology who studies China’s ties with its neighbors, referring to the incident between China and Vietnam.

China has doubled defense spending since 2006, with Xi pushing through a 12 percent jump for next year as he seeks to build a “blue-water” navy capable of operating on the high seas and far from home ports. It has also escalated tensions in the East China Sea after establishing an air defense identification zone over islands disputed with Japan.

U.S. President Barack Obama last month visited the Philippines and Japan during a tour aimed at reassuring Asian allies of U.S. support in the face of China’s rising economic and military power. China’s Foreign Ministry spokeswoman Hua Chunying yesterday called the U.S. “irresponsible” after her American counterpart Jen Psaki said in Washington the move to establish a rig near the Paracels was “provocative and unhelpful.”

China’s “unilateral action appears to be part of a broader pattern of Chinese behavior to advance its claims over disputed territory in a manner that undermines peace and stability in the region,” Psaki said separately in a statement yesterday. “We are also very concerned about dangerous conduct and intimidation by vessels operating in this area.”

The confrontation off Vietnam’s coast, where China National Offshore Oil Corp. placed an exploration rig May 2, is the most serious between the two countries since 2007 when Chinese naval patrol vessels fired on a Vietnamese fishing boat, killing one sailor. In 1988, a Chinese naval attack in the Spratlys, which Vietnam also lays claims to, killed 64 Vietnamese border guards as China seized seven atolls. The two nations fought a war after China invaded Vietnam in 1979.

China has 80 vessels in the area, including seven military craft, some of which fired water at Vietnamese ships backed by low-flying Chinese aircraft, Ngo Ngoc Thu, Vice Commander of Vietnam’s Coast Guard said at a briefing. Six Vietnamese officers were hurt by broken glass during the clash, he said.

“The situation is extremely tense,” Thu said. While Vietnam seeks to resolve the dispute through negotiations, “all endurance has limits.” “If China vessels continue to hit ours, we will have similar moves to respond in self-defense.”

Vietnam earlier protested the placement of the HD-981 rig and deployment of vessels in an area it considers part of the country’s exclusive economic zone and continental shelf, Deputy Prime Minister Pham Binh Minh told Chinese State Councilor Yang Jiechi May 6 by phone, according to a posting on the website of Vietnam’s foreign ministry. Vietnam demanded China withdraw the rig and vessels and hold talks to resolve the issue, Minh said.

The number of Chinese ships being reported in the area signals “a major show of resolve,” Ian Storey, a senior fellow at the Institute of Southeast Asian Studies in Singapore, said by e-mail. “Vietnam seems equally keen to demonstrate its determination to prevent the rig from carrying out its work.”

The rig’s location is near the Paracel Islands, which are now under Chinese control, said Li Mingjiang, an associate professor at the S. Rajaratnam School of International Studies in Singapore. Vietnam also claims the Paracels and both countries, as well as Association of Southeast Asian Nations members Brunei, Malaysia and the Philippines, have claims to other territory in the South China Sea.

“Vietnam will openly criticize China in the international arena and mobilize other Asean countries, especially those claimant countries, to put pressure on China,” Li said yesterday by e-mail, before news broke of the collision.

Near the Spratlys, the Chinese fishing vessel and its crew were detained by Philippine authorities, the Foreign Ministry’s Hua said.

The Philippine Department of Foreign Affairs said the crew was arrested to “enforce maritime laws and to uphold Philippine sovereign rights.” The government will handle the case in a “just, humane and expeditious manner” after the fishermen were found with a large number of endangered species, it said by e-mail.

“China has indisputable sovereignty over the Nansha Islands and its adjacent waters including the barrier reefs,” Hua said, using the Chinese name for the Spratlys. “We ask the Philippine side to release the vessel and the crew. And we urge the Philippine side to stop taking further provocative actions.”

The Philippines in March called China a “threat to our security” as it protested Chinese attempts to prevent resupply of a Philippine vessel in the area.

“It shows the regional concern that China has yet to agree or commit to a timetable with respect to the code of conduct — frustration that China hasn’t done that,” said Terence Lee, an assistant professor of political science at the National University of Singapore, referring to the Philippine and Vietnam actions. “Finding some way forward concretely without the use of force is imperative for the countries in the region.”

Asean is seeking a code for the oil- and gas-rich waters in the South China Sea, through which some of the world’s busiest shipping lanes run. The talks have made little progress since China agreed last July to start discussions, before introducing fishing rules in January requiring foreign vessels to seek permission before entering waters off its southern coast.

China has said it is prepared to hold bilateral talks over territory and rejects a Philippine move for international arbitration.

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