SHANGHAI (AFP) - A US debt default could spur China to diversify its multi-trillion-dollar foreign exchange reserves, the world's largest, analysts say, as Beijing seeks to boost its voice in the global economy.

A historic default would inevitably lower the value of China's US dollar assets and have a broader impact on its economy, the second-biggest in the world.

Chinese officials and state media have already sounded the alarm as the deadline approaches for the divided US Congress to raise the debt ceiling and avoid the financial catastrophe.

Analysts said the heightened possibility of default is likely to cause China to further diversify its reserves - which it is already doing - and even trim holdings of US Treasuries, but they ruled out Beijing dumping its holdings.

The bulk of China's foreign exchange reserves, which reached a whopping US$3.66 trillion (S$ 4.6 trillion) by the end of last month, are held in the greenback.

"If there really is a default, the Chinese government will definitely speed up foreign exchange reserve diversification, seeking safer bonds of other countries," said Liao Qun, Hong Kong-based economist for Citic Bank International.

"If there is an acceleration in diversifying, there might also be a reduction in holdings (of US Treasuries)," he told AFP. "China has a reason to do so, but of course it would be extremely difficult."

China is the largest foreign holder of US Treasury bonds with US$1.28 trillion, closely followed by Japan at US$1.14 trillion.

Any large-scale selling of US debt by China would erode the value of its remaining holdings, analysts said, while few other asset classes anywhere in the world were large enough to park such vast amounts of cash, leaving Beijing with few practical alternatives.