China, the top holder of US Treasury bonds, has urged the United States to adopt 'responsible measures' after ratings agency Standard Poor's cut the outlook on US sovereign debt to negative.
SP sent stocks plunging worldwide when it slashed its outlook from 'stable' to 'negative' on Monday, pointing to doubts about Washington's ability to tackle looming debt and fiscal deficits -- concerns raised by Beijing in the past.
'US Treasury bonds are a reflection of US government credit and are important investment products for domestic and international institutional investors,' foreign ministry spokesman Hong Lei said in a statement.
'We hope the US government will earnestly adopt responsible policy measures to guarantee the interests of investors,' he added.
SP on Monday gave the United States until 2013 to come up with a credible plan for addressing its financial problems, or risk losing its coveted 'AAA' credit rating which helps it to borrow at ultra-low rates.
The US government immediately rebuffed SP, saying the powerful rater, whose move sent US borrowing costs sharply higher, 'underestimated' the country's ability to tackle the problem.
'We believe SP's negative outlook underestimates the ability of America's leaders to come together to address the difficult fiscal challenges facing the nation,' the Treasury Department said in a statement.
But the announcement sent global markets falling, including in China where the Shanghai Composite Index dropped 1.91 per cent on Tuesday in what dealers said was the after-effect of the SP warning.
China is the top holder of US debt, and had $US868.4 billion ($A829.26 billion) invested in US Treasuries at the end of August last year, US data showed.
Beijing has in the past raised worries that the massive US stimulus effort launched to revive the economy would lead to mushrooming debt that would erode the value of the dollar and its Treasury holdings.
Mark Williams, a London-based economist at Capital Economics, said the downgrade would reinforce these concerns.
It 'will strengthen the belief among Chinese officials that US policymakers cannot be trusted to safeguard the value of China's dollar investments and that the dollar-centred global financial system needs reform,' he said.
But he added that Beijing had few other alternatives.
'Beijing's hands are tied. Its still-massive foreign exchange accumulation forces it to invest over $US50 billion ($A47.75 billion) outside China each month,' he said.
'A large chunk of that will continue to go into US Treasuries because there are few big enough alternative markets.'
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