* U.S. urges China to join “principled security network”

* Carter says U.S. to remain main guarantor of security

* Japan pledges to help build SE Asian security capacity (Recasts, adds comment from Indian, Malaysian defence ministers)

SINGAPORE, June 4 (Reuters) - China came under pressure from the United States and Asian powers to rein in its actions in the South China Sea, with the U.S. defense secretary urging Beijing to join in on regional cooperation or risk erecting a “Great Wall of self-isolation.”

U.S. Defense Secretary Ash Carter encouraged China to participate in a “principled security network” for Asia to help counter concerns about its strategic intentions following “expansive and unprecedented actions” in the South China Sea.

Carter also told the Shangri-La Dialogue, a regional security forum in Singapore, that the United States would remain the main guarantor of regional security for decades to come and warned China against provocative behaviour.

Any action by China to reclaim land in the Scarborough Shoal, an outcrop in the disputed South China Sea, would have consequences, Carter said.

“I hope that this development doesn’t occur, because it will result in actions being taken by the both United States and ... by others in the region which would have the effect of not only increasing tensions but isolating China.”

The South China Sea has become a flashpoint between the United States, which increased its focus on the Asia-Pacific under President Barack Obama’s “pivot”, and China, which is projecting ever greater economic, political and military power in the region.

“The situation in the South China Sea continues to be viewed with concern,” Indian Defence Minister Manohar Parrikar told the forum. “All countries in the region need to recognise that our shared prosperities and the enviable rate of growth that this region enjoys over past decades will be put at risk by aggressive behaviour or actions by any one of us.”

The United States and many Asian countries were stepping up security cooperation to ensure they were able to make choices “free from coercion and intimidation,” Carter said.

“Even as the United States will remain the most powerful military and main underwriter of security in the region for decades to come - and there should be no doubt about that - those growing bilateral relationships demonstrate that nations around the region are also committed to doing more to promote continued regional security and prosperity,” he said.

Japanese Defence Minister Gen Nakatani, speaking at the same event, said his country would help Southeast Asian nations build their security capabilities to deal with what he called unilateral, dangerous and coercive actions in the South China Sea.

“In the South China Sea, we have been witnessing large-scale and rapid land reclamation, building of outposts and utilization of them for military purposes,” Nakatani said, without mentioning China directly. “No countries can be an outsider of this issue.”

MOST POWERFUL

Trillions of dollars of trade a year passes through the South China Sea, which is home to rich oil, gas and fishing resources. Vietnam, the Philippines, Malaysia, Brunei and Taiwan have claims in the area, and rising tensions have been fuelling a rising in security spending in the region.

“The uncertainty of China’s future trajectory is arguably the main driving concern about possible military competition now and in the future,” Malaysian Defence Minister Hishammuddin Hussein said.

Carter said that for decades some critics had been predicting an impending U.S. withdrawal from the region, but this would not happen.

“That’s because this region, which is home to nearly half the world’s population and nearly half the global economy, remains the most consequential for America’s own security and prosperity.”

In an apparent counter to “America-first” policies expounded by prospective Republican presidential candidate Donald Trump, including suggestions that U.S. troops should be withdrawn from Asia, Carter stressed bipartisan support for continued engagement.

“Regardless of what else was going on at home or in other parts of the world - during Democratic and Republican administrations, in times of surplus and deficit, war and peace - the United States has remained economically, politically, and militarily engaged, as well as geographically located in the Asia-Pacific,” he said.

The Shangri-La Dialogue is being held ahead of a significant ruling expected in coming weeks on a case filed by the Philippines in the International Court of Arbitration challenging China’s South China Sea claims, which Beijing has vowed to ignore.

The United States has been lobbying Asian and other countries to back the judges’ statement that their ruling must be binding, a call echoed by Japan on Saturday.

China has lobbied on the other side for support for its position that the court lacks jurisdiction in the case. (Additional reporting by Masayuki Kitano and Paige Lim; Editing by Raju Gopalakrishnan and Lincoln Feast)