BEIJING (REUTERS) - In one of the many frank exchanges US Defence Secretary Chuck Hagel had in China this week, General Fan Changlong told him how one of his uncles died as a slave in a Japanese mine during World War Two.

General Fan, the deputy head of China's powerful Central Military Commission, spoke about the lessons of history, signalling Beijing's concerns that the United States was siding with Japan against China.

Mr Hagel replied by saying his own father had helped fight Japanese forces in World War Two.

"The secretary made it very clear that we should be informed by history but not driven by it," a US official told Reuters, speaking on condition of anonymity to recount a conversation on Tuesday that he described as terse.

The exchange sums up the frustration in China over America's role in Asia, where in the eyes of Beijing, Washington is increasingly supporting Japan as well as other countries over territorial disputes with China.

The United States has said it is not taking sides but stands ready to defend its allies.

China, some experts said, appeared to be getting anxious that recent tough talk from US officials over the disputed East and South China Seas could be a preview of what US President Barack Obama would say when he visits Asia this month.

Dispensing with diplomatic protocol, China has made clear this week that it does not want Mr Obama jumping in with both feet when he travels to Japan, the Philippines and Malaysia.

While Beijing has territorial disputes with all three, its ties with Japan and the Philippines, both US allies, are in the deep freeze.

Mr Obama will also visit South Korea, with whom Beijing is enjoying warm relations.

China is at loggerheads with Japan in the East China Sea over uninhabited islets that are administered by Tokyo. China also claims most of the South China Sea. The Philippines, Malaysia, Vietnam, Brunei and Taiwan claim parts of those waters.

"Obama needs to pay serious consideration to this issue when he comes to Asia...China has already put this message across during the meetings with Hagel," said Mr Ruan Zongze, a former diplomat with the China Institute of International Studies in Beijing, a thinktank linked to the Foreign Ministry.

"The United States is moving in a direction we don't want to see, taking sides with Japan and the Philippines, and China is extremely unhappy about this."

An Obama administration official acknowledged to reporters traveling with Mr Hagel that the tone was sharper on issues surrounding the South and East China Seas than it had been on the last visit by a US defence secretary to China. That was when Mr Hagel's predecessor Leon Panetta visited in 2012.

"But in other areas the tone was actually improved," the Obama administration official said, pointing to discussions on Sino-US military cooperation and even North Korea.

CHINA DISSATISFIED

On Tuesday, Chinese Defence Minister Chang Wanquan told Hagel that Washington should restrain Japan and chided the Philippines.

General Fan told Hagel outright that the "Chinese people are dissatisfied" with US support for Japan and South-east Asia, according to a statement carried on the Chinese defence ministry's website.

The influential tabloid the Global Times, published by the Communist Party's official People's Daily, said in an editorial on Wednesday that such strong words "have not been seen much in the past".

China's ties with Japan have long been poisoned by what Beijing sees as Tokyo's failure to atone for its occupation of parts of China before and during World War Two.

Japan's repeated official apologies for wartime suffering are sometimes undercut by contradictory comments by conservative politicians.

For its part, China has alarmed the region, and Washington, with its increasingly tough line on territorial disputes.

It announced its biggest rise in military spending in three years last month, a signal from President Xi Jinping that China is not about to back away from its growing assertiveness.

China's military spending has allowed Beijing to create a modern force that is projecting power not only across the disputed waters of the East and South China Seas, but further into the western Pacific and Indian Oceans.

Mr Jia Qingguo, dean of the School of International Studies at Peking University and who has advised the government on diplomatic issues, said the last thing Beijing wanted was for Mr Obama to round on China when he is in Asia.

"They (Chinese leaders) hope that the Obama visit will not be used to rally other countries against China. If you listen to the harsh rhetoric of senior (US) administration officials, this is a genuine concern." Much of the tough comments from US officials have come since China announced the creation of a controversial new air defence identification zone that covers the disputed Japanese-administered islands in the East China Sea.

"They (Chinese officials) are trying to figure out whether it's the lower level (US) people coming out and making these comments so the boss doesn't have to, or whether it's moving to a crescendo," said Mr Christopher Johnson, a former senior China analyst at the CIA and now at the Center for Strategic and International Studies in Washington.

"I think there is a concern that this debate could be swayed substantially if Obama were to make very forthright comments on this trip and that could tip the balance internally and make it more difficult for Xi to emphasise the Sino-US relationship as paramount."