BEIJING » China’s planting of an oil platform in contested waters off Vietnam drew strident complaints from Hanoi, a messy standoff between ships and violent protests among Vietnamese — but nothing to dislodge the rig and no broader pushback in the region.

Southeast Asian countries, with diverging interests and wariness of angering Beijing, are shunning any collective action that might halt China as it relentlessly nudges forward its sovereignty claims in disputed seas seen as a possible flashpoint for the world’s next major conflict.

Despite its accusations of Chinese bullying, Vietnam can expect little in the way of outside help as its patrol boats continue to spar with Chinese vessels guarding the rig in the South China Sea.

"The divisions already existed (among Southeast Asian countries), but China is very adept at exploiting them," said Ian Storey, an expert on regional politics at the Institute of Southeast Asian Studies in Singapore.

"At the end of the day, Vietnam is on its own," Storey said.

The lack of unity encourages China as it looks to cement its claim to virtually the entire South China Sea, its island groups and its maritime wealth — including potentially significant deposits of petroleum needed to keep the Chinese economy booming.

China calibrates the pitch of its assertiveness depending on surrounding events and the amount of push-back it receives. So far, its actions have mainly targeted the Philippines and Vietnam, while other countries that also claim parts of the South China Sea such as Malaysia and Brunei are left alone. To avoid escalating matters too quickly, China generally relies on its coast guard rather than the navy when confronting ships of other nations.

It isn’t clear why China chose May 1 to move the rig from the state-run China National Offshore Oil Corporation into position about 20 miles from the China-controlled Paracel Islands and 173 miles from the coast of Vietnam.

While China says that’s simply part of its ongoing search for resources, some have speculated it was a deliberate test of Vietnamese resolve and a warning to Hanoi against closer security ties with the Beijing’s main rival, the U.S.

"It seems to be a put-up-or-shut-up move," said Carl Thayer, a Vietnam and regional security expert at Australia’s University of New South Wales.

China’s action was met with immediate, though apparently fruitless, opposition by Vietnam, which also claims the Paracels and says the rig is inside its exclusive economic zone.

Hanoi sent ships to harry Chinese craft protecting the rig.

Anti-Chinese anger, ever-present in Vietnam, bubbled to the surface last week in violent attacks that left at least two Chinese workers dead and 140 injured. Thousands of Chinese have since been evacuated by sea and air.

The latest confrontation is among several Chinese moves bolstering its hold on the South China Sea since around 2008. China has expelled Philippine fishing boats from reefs and atolls, built scattered military outposts, demanded that foreign countries apply for permission to fish in the area, and dispatched a naval flotilla to reassert Chinese sovereignty over James Shoal off the coast of Borneo — a full 930 miles south of China’s island province of Hainan.

Despite scattered protests and steps by its neighbors to shore up their own presence in the area, nothing has effectively impeded China’s progress.

Storey said both the Philippines and Vietnam dearly desire the backing of their fellow members of the Association of Southeast Asian Nations in their disputes with China. The grouping had shown some degree of unity in the 1990s, closing ranks behind the Philippines in an earlier territorial dispute with China, he said.

However China’s growing clout, politically and economically, has sapped the group’s resolve. So has the entry into ASEAN of Laos, Cambodia and Myanmar, all of which have strong ties to Beijing and no direct stake in the South China Sea dispute, Storey said.

This month’s ASEAN summit, about a week after China installed its rig off Vietnam’s coast, expressed concern about maritime disputes but did not even mention China by name.

Some Southeast Asian countries also may want to stay out of what they suspect are moves that are actually directed at the U.S., which has been increasingly critical of what it describes as Chinese provocations, said Tan See Seng, of Singapore’s Nanyang Technological University.

China chafes at U.S. dominance, including its security alliances with the Philippines and others, and has long sought to curtail U.S. intelligence gathering and military operations in the South China Sea.

Washington’s moves to beef up its presence in Asia after a decade of war in the Middle East have particularly riled Beijing, which says that is emboldening its neighbors and raising tensions.

"Why draw unwanted attention to oneself if a backlash only strengthens Chinese suspicions that one is indeed in cahoots with the Americans," Tan said.

So far, the U.S. has offered mere rhetorical support for Beijing’s rivals, saying issues must be resolved peacefully and without hindering navigation.

"We just need to cool off, move in a deliberate manner and hopefully solve this diplomatically," U.S. Chief of Naval Operations Adm. Jonathan Greenert said Monday when asked about the China-Vietnam dispute.

Such statements pale in comparison to strong U.S. assertions of support for treaty partner Japan, with whom China is engaged in a dangerous feud over uninhabited islands in the East China Sea that are controlled by Japan but claimed by both.

China may be hurting its reputation by being seen as bullying smaller countries in a region where it wants to be seen as a benign regional overlord that will one day replace the U.S.

Yet Beijing apparently has calculated that strong trade and investment ties with the region will head off any major rift, Tan said.

"China seems prepared to absorb any short-term costs its actions might incur for what it perceives is the fundamental strategic gain of ensuring its rise is not unduly, and — in its view — unfairly constrained by the U.S. and its partners," Tan said.

Although China says its oil rig will cease drilling at the start of typhoon season in August, Beijing seems likely to keep raising the stakes in the South China Sea.

One way would be by declaring an air defense zone over all or part of the area, similar to what it did last year over a wide swath of the East China Sea. Storey called the move "only a matter of time."