For the first time in 11 months China's holdings of US Treasury bonds fell - to $763.5 billion in April, US government data showed.
The figure, down from March's $767.9 billion, was the lowest since June 2008.
They do not include US Treasury bond holding in Hong Kong Special Administrative Region, which climbed to $80.9 billion in April from $78.9 billion the previous month.
The decline in the China holding "seems to stem from net selling of Treasury bills," said Chirag Mirani of Barclays Capital Research.
On the whole, foreigners decreased holdings of Treasury bills by $44.5 billion in April, the data showed.
As the largest holder of US Treasury bills, which are crucial to funding Washington's multi-trillion-dollar recovery plans, China had expressed concerns recently over what it called the safety of its dollar-linked assets.
US Treasury Secretary Timothy Geithner traveled to Beijing about two weeks ago to reassure Chinese leaders, saying their money is "very safe" despite the US budget deficit, which he pledged to cut.
The United States has been running large budget shortfalls since the tenure of Democratic President Barack Obama's Republican predecessor George W. Bush.
Obama administration officials estimate a deficit of $1.841 trillion for the 2009 budget and $1.258 trillion in 2010.