China’s military presence in the South China Sea has already garnered criticism from the Philippines and Vietnam. As the Philippines expresses concern over China’s military presence, the United States is now accusing China of raising tensions, from Aljazeera:

The United States has accused China of raising tensions through a new military garrison in the South China Sea, calling on all sides to lower tensions in the hotly contested waters.

“We are concerned by the increase in tensions in the South China Sea and are monitoring the situation closely,” US State Department spokesman Patrick Ventrell said in a statement on Friday.

“In particular, China's upgrading of the administrative level of Sansha city and establishment of a new military garrison there covering disputed areas of the South China Sea run counter to collaborative diplomatic efforts to resolve differences and risk further escalating tensions in the region,” he said.

Ventrell also pointed to “confrontational rhetoric” and incidents at sea, saying: “The United States urges all parties to take steps to lower tensions.”

Washington has voiced it’s support for the Philippines in the past, and analysts, such as Stanford University’s Don Emmerson, are speculating on the US’s role in the dispute, the Voice of America reports:

“One would even suggest that those within the People’s Liberation Army who are among the most vehement nationalists on this issue would like to see the South China Sea actually become a Chinese lake,” Emmerson says.

Cato’s Justin Logan believes Washington is going “a little bit too far into making this a U.S. vs. China competition in the South China Sea.”

“If the United States had a somewhat more distant posture and wasn’t always rushing to assure its friends and allies in the region that we would be on the hook to ensure freedom of navigation,” Logan says, “other countries in the region would be, in fact, more alarmed about China’s behavior.”

“But the idea that they see the United States at the forefront of this effort to constrain China’s ambition in the South China Sea allows them to stand back a little bit and play one side off the other.”

Tensions in the region were heightened due to the lack of a resolution from the ASEAN summit. In response to Washington, Xinhua reports China strongly opposes the US’s statement on the dispute. :

China expressed its strong dissatisfaction and resolute opposition on the press statement released Friday by the U.S.Department of State on the South China Sea, Foreign Ministry spokesman Qin Gang said here Saturday.

The statement “completely ignored the facts, deliberately confounded right and wrong, and sent a seriously wrong signal, which is not conducive to the efforts safeguarding the peace and stability of the South China Sea and the Asia Pacific region,” Qin said in a statement.

Why does the U.S. turn a blind eye to the facts that certain countries opened a number of oil and gas blocks, and issued domestic laws illegally appropriating Chinese islands and waters? Why does the U.S. avoid talking about the threats of military vessels to Chinese fishermen by certain countries and their unjustified claims of sovereignty rights over Chinese islands? And why do they choose to abruptly express concerns on and fiddle with the issue at a moment when regional countries are enhancing communication and dialogue and trying to solve the disputes and calm the situation? Qin asked.

China attached great importance to the friendly cooperation with ASEAN, support the process of ASEAN integration and ASEAN' s major role in the East Asia cooperation, said Qin.

Reuters reports that China protested the U.S. accusations by calling in the U.S. Diplomat, Robert Wang:

In a statement released late on Saturday, China's Foreign Ministry said Assistant Foreign Minister Zhang Kunsheng summoned the U.S. Embassy's Deputy Chief of Mission Robert Wang to make “serious representations” about the issue.

Zhang said the U.S. statement “disregarded the facts, confused right with wrong, sent a seriously wrong signal and did not help with efforts by relevant parties to maintain peace and stability in the South China Sea or the Asia Pacific.”

In the Vietnamese capital Hanoi, police on Sunday arrested at least 40 people preparing to stage another anti-China protest, said blogger Phuong Bich.

Authorities in Vietnam rarely allow demonstrations, but there have been periodic protests against China in Hanoi over the past few months as a result of what many Vietnamese see as Beijing's aggressive moves in the South China Sea

According to Bloomberg, China’s assertiveness in the region coincides with the upcoming change in leadership:

“What we’re seeing is a significant ratcheting up of Chinese pressure on the region to basically acquiesce that the South China Sea is Chinese territory,” Dean Cheng, a researcher on Chinese political and security issues at the Heritage Foundation in Washington, said in a telephone interview.

China’s assertiveness in the region coincides with preparations for a political and military transition. Communist party leaders are believed to be meeting to select new members for the country’s Politburo and Standing Committee, the nation’s civilian leaders, and its Central Military Commission, which controls the military, Cheng said. In October or November, the full party Congress will convene to select a new president to replace Hu Jintao in a once-a-decade leadership handover.

Cheng said the U.S. interest is primarily in free navigation through the region. The South China Sea sea lanes are particularly important to the economies of U.S. allies Japan and South Korea, he said.

Any military conflict in the South China Sea might draw in the U.S. The Philippines is a U.S. ally, and the two countries have a mutual protection pact. While the treaty doesn’t cover disputed territories in the South China Sea, an attack on Filipino ships or other assets could trigger a U.S. response, Cheng said .

China is becoming more aggressive in the region by setting up the city of Sansha and a military garrison. Another Reuters article reports they are also increasing their presence through China National Offshore Oil Corp’s (CNOOC) acquisition of Canada’s tenth largest oil company, Nexen:

The acquisition of Nexen — which faces reviews by both the Canadian and U.S. governments — will not be an instant game changer, however, because deepwater technology will take years for China to master, say experts on China's energy sector.

CNOOC, parent of Hong Kong-listed CNOOC Ltd, has emerged as a key component of China's strategy to bolster its claims to nearly the entire South China Sea. The vast body of water, believed to hold rich oil and gas reserves, is also claimed in part by Vietnam, the Philippines, Taiwan, Brunei and Malaysia.

CNOOC become more deeply involved in China's South China Sea strategy in late June when it invited foreign firms to bid on oil blocks that overlap territory being explored by Vietnam. Earlier CNOOC unveiled the Haiyang Shiyou (Offshore Oil) 981 rig, China's first domestically made ultra-deepwater rig.

With the Nexen acquisition comes deepwater fields in the Gulf of Mexico that would bring with it expertise in managing complex operations and technology that, once assimilated, would extend CNO OC's rea ch in the South China Sea.