China sending an oil rig to waters disputed with Vietnam is a move to assert its legal claim and practical hold over contested territory whatever the short-term political and diplomatic costs, analysts say -- but could play into Washington's hands.

Beijing's controversial move to dispatch the deep-water rig along with a reported 70 vessels triggered clashes in the South China Sea, just after a visit to the region by US President Barack Obama and ahead of this weekend's Association of Southeast Asian Nations (ASEAN) summit.

It also comes amid heightened tensions between Beijing and Manila, which has asked a United Nations tribunal to rule on China's claims over most of the sea. Beijing -- which prefers to negotiate directly with its smaller, weaker neighbours -- has vehemently rejected arbitration.

Experts say that while Beijing has cast the drilling operation by state-owned CNOOC as part of its long-term oil exploration programme, energy resources are probably a secondary consideration.

Rather, they note, the move appears to be a fresh effort by China to demonstrate a so-called "incident of sovereignty", part of a broader strategy geared towards showing Beijing has control of disputed territory.

"I think that the Chinese government is trying to be assertive with regard to its claims about this or that little island in the South China or East China Seas in order to keep those claims alive," said Barry Sautman, a specialist on Chinese politics at the Hong Kong University of Science and Technology (HKUST).

"Under international law... all states that have territorial disputes must periodically do something to show that they have an active interest in the territory in question," he said.

"Whether this is politically beneficial to China is, of course, another matter," he added.

Beijing has defended its actions as "completely reasonable, legal and justified", arguing that the intended drilling location is close to the Paracel Islands.

China has controlled the archipelago since ousting South Vietnamese forces in 1974 but Hanoi still claims them.

-- Caught between politics and law --

Vietnam says Chinese boats have used water cannon and repeatedly collided with Hanoi's patrol ships since May 3, injuring six people, while Beijing counters that "disruptive" Vietnamese vessels have rammed its ships 171 times,

The clash is only one of several maritime spats between China and its Asian neighbours, the most volatile of them with Japan over a small East China Sea island grouping called Diaoyu by Beijing and Senkaku by Tokyo.

In that dispute, a key Japanese argument is that until the 1970s, "there was a substantial period in which China didn't show any interest" in the islands, Sautman said, and Beijing will want to avoid allowing that argument elsewhere.

"I think China is caught in between the requirements of politics and the requirements of law," Sautman said.

The timing of Beijing's move "has spurred speculation that this was a tit-for-tat response" to Obama's trip to US allies Japan, South Korea, Malaysia and the Philippines last month, when China's territorial claims were a constant theme, noted Carl Thayer, a Vietnam expert and emeritus professor at the Australian Defence Force Academy.

But the unilateral step -- similar to Beijing's November declaration of an "air defence identification zone" over much of the East China Sea -- risks bolstering Washington's argument that China is taking "provocative" steps in the region.

"If I were an American, I'd say, 'Thank you', because it just makes everybody feel that China's being aggressive," said David Zweig, director of HKUST's Centre on China's Transnational Relations.

"It's clear, there's tension in all of this area and it wasn't happening before China was stronger," he added.

-- 'Unexpected, provocative and even illegal' --

Nonetheless Beijing's latest action comes as a surprise because it represents "a marked reversal in the trajectory of bilateral relations" between Communist neighbours China and Vietnam, said Thayer.

The two fought a brief border war in 1979, but ties had been at a high point in recent months following Chinese Premier Li Keqiang's visit to Hanoi last October and a pledge by both countries to boost two-way trade to $60 billion by 2015.

China's drilling announcement came "out of the blue" and was "unexpected, provocative and even illegal", said Thayer.

The assertiveness of the move against Vietnam suggests that Beijing's current leadership under President Xi Jinping is "more inclined to adopt a heavy-handed approach" than previous generations, said Li Mingjiang, an expert on East Asian security and an associate professor at the S. Rajaratnam School of International Studies in Singapore.

But Hanoi's strong reaction could suggest that regional attitudes towards China are hardening, he added -- something that Beijing appears not to have anticipated.

"(China) probably didn't calculate that Vietnam would actually send all these ships to try to make China remove the rig," Li said.

Hanoi had to avoid sending a signal of tacit acquiescence to Beijing, he added.

"Basically, I think Vietnam just cannot afford the consequences of not reacting strongly."