By John O'Callaghan and David Alexander

SINGAPORE (Reuters) - Senior Chinese military officials came ready to talk at a major regional security forum over the weekend, surprising delegates with a new sense of openness at a time when Beijing is making strident claims to territory across Asia's seas.

No one expected any resolution of disputes over maritime boundaries, accusations of Chinese cyber-espionage, Beijing's suspicions about the U.S. "pivot" to Asia or other prickly issues at the annual Shangri-La Dialogue in Singapore.

But the charm offensive by the People's Liberation Army (PLA) officers, less than a week before Chinese President Xi Jinping meets U.S. President Barack Obama for an informal summit, appeared to be designed to tone down the recent assertiveness by emphasizing cooperation and discussion.

"There's no question that this year the PLA delegation has come very prepared to engage in dialogue," said John Chipman, director-general of the London-based International Institute for Strategic Studies, which convenes the forum. "The intensity of the Chinese engagement and the manner of their engagement is different."

The defense minister of the Philippines, Voltaire Gazmin, also noted a shift.

"It's a total turnaround. They have been talking about peaceful resolutions, no outward acts," Gazmin told Reuters. "But we still hope to see that these words are put into action."

China claims large swathes of the South China Sea, which could be rich in oil and gas. The Philippines and other Southeast Asia nations have challenged Beijing over those claims.

Beijing is also embroiled in a row with Tokyo over uninhabited islands in the East China Sea, which are also believed to contain large energy deposits.

China, the world's second-largest economy and a rising military power, is aware it needs what it calls a "stable and peaceful external environment" for its own development.

Indeed, Chinese officials at the forum sought to ease concerns about Beijing's intentions.

"China's development and prosperity is a major opportunity instead of a challenge or even threat to countries in the Asia-Pacific region," Lieutenant General Qi Jianguo, the PLA's deputy chief of general staff, told a session on regional security.

Qi, China's top official at the forum, said dialogue "by no means denotes unconditional compromise" and he gave no ground on sovereignty claims, calling the presence of Chinese warships in the East China Sea and the South China Sea "totally legitimate and uncontroversial to patrol within our own territory".

But he said "China is a peace-loving nation" and went on to answer more than a dozen questions from delegates.

PLAYING THE GAME

Unlike most other countries, China has sent its defense minister to the Shangri-La Dialogue only once - in 2011.

Despite that absence, a senior U.S. official accompanying Defense Secretary Chuck Hagel to the forum saw a big change in the Chinese delegation.

"Last year China had a very, very small contingent, a relatively junior-ranking contingent. This year they came in force ... and have been very active in the panels," said the official. "That's very, very good. We want everybody to engage."

While there was a fair amount of skepticism about China's position from security analysts during the various sessions, Chinese officials were not shy about taking tough questions or asking their own from the floor.

Major General Yao Yunzhu from the PLA's Academy of Military Science asked Hagel after his speech how Washington could reassure Beijing that the U.S. focus on Asia was not an "attempt to counter China's rising influence".

"China is not convinced," she said in fluent English.

"That's really the whole point behind closer military-to-military relationships," Hagel replied. "We don't want miscalculations and misunderstandings and misinterpretations."

The higher-ranking Chinese delegation this year and their participation in the sessions shows "a more active effort on the part of the Chinese to reach out", Canada's defense minister, Peter MacKay, told Reuters. "I see that as positive."

The Chinese worked "with a very courteous style, with a much less combative style", Chipman said, noting the remarks by "a young officer of the PLA congratulating the defense minister of Japan for his very important and serious speech".

Japan, a U.S. ally, is strengthening its economy and military to play a responsible international role, Defense Minister Itsunori Onodera said in his speech.

Onodera, addressing lingering suspicion about his country's intentions given its role in World War Two, said Japan "caused tremendous damage and suffering" to its neighbors in the past but wanted to look to the future by promoting cooperation.

Those comments were what won public praise from the PLA officer, who also spoke in English.

"The other Asians are saying the Chinese have decided to play the game, that is to pitch up, make an impression and do so in the right way," said Chipman.

"How that has an impact on the ground, at sea, in space, in cyber ... is a different question."

(Additional reporting by Ben Blanchard in BEIJING and Ron Popeski in SINGAPORE. Editing by Dean Yates)