By John O'Callaghan

SINGAPORE (Reuters) - Vietnam's prime minister called for unity among Southeast Asian countries as China asserts its claims to the energy-rich South China Sea, warning that any conflict could disrupt international trade and the global economy.

Tensions in the decades-old territorial dispute between six Asian claimants have risen in recent weeks after Chinese vessels converged near a ship the Philippines ran aground on a reef in 1999 to mark its territory.

"Somewhere in the region, there have emerged preferences for unilateral might, groundless claims and actions that run counter to international law and stem from imposition and power politics," Nguyen Tan Dung said in a speech on Friday at the Shangri-La Dialogue, an annual regional security forum in Singapore.

"A single irresponsible action or instigation of conflict could well lead to the interruption of huge trade flows, causing unforeseeable consequences not only to regional economies but also to the entire world," he said in remarks translated from Vietnamese.

China claims almost all of the South China Sea as its territory, setting it directly against the Philippines and Vietnam as it displays the growing "blue water" reach of its navy and the United States turns more of its attention to Asia.

Brunei, Taiwan and Malaysia also claim parts of the South China Sea, whose waters are vital to the international flow of goods and energy and whose seabed is believed to contain rich deposits of oil and natural gas.

U.S. Defense Secretary Chuck Hagel is attending the three-day forum convened by the International Institute for Strategic Studies (IISS), with the U.S. "pivot" toward Asia, the region's military build-up and the South China Sea high on the agenda.

Stressing the need for "strategic trust", Dung said the Association of Southeast Asian Nations (ASEAN) must stay united and strong, without any of its 10 members "forced to take sides with one country or the other for the benefit of their own relationships with big powers."

ASEAN has been talking to China about a binding code of conduct to ease tensions but Beijing has said it will negotiate "when the time is ripe". ASEAN foreign ministers are due to meet in Thailand in August to forge a position on the code of conduct before meeting Chinese officials in Beijing.

Vietnam will not be a military ally to anyone or allow any country to set up military bases on its soil, Dung said, adding the modernisation of its forces was "only for self-defence and to safeguard our legitimate interests".

"OPPORTUNISTIC ASSERTIVENESS"

China's response to the actions of its rival claimants may be part of a very long-term negotiating strategy, said Christian Le Miere, a senior fellow at the IISS.

"I would call it a form of opportunistic assertiveness whereby China is often aware that these actions are going to happen and then uses them as a justification for its overzealous reactions," Le Miere told a news conference.

"What we will continue to see is China trying to change the facts on the water and trying to build a stronger legal case and adapt the legal environment to its own benefit wherever possible and continue with its maximalist claims because they will, in the future, provide China with a stronger negotiation position."

A Chinese military think-tank, the Centre for National Defence Policy, said this week the U.S. pivot to Asia had "shattered" the relative calm of the South China Sea.

"While the conditions do not yet exist for a large-scale armed clash, the dispute is becoming normalised and long-term ... and ineffective management may lead to a serious crisis," the report said, according to the China News Service.

Washington says it is focusing more security, economic and diplomatic attention on Asia to engage the fast-growing region, which has fuelled Chinese suspicions that the United States is trying to contain its economic and military might.

(Additional reporting by Kevin Lim; Editing by Sonya Hepinstall)