China's politically sensitive trade surplus fell to $14.51 billion in September as exports slowed sharply, hit by economic turmoil in the United States and Europe, official data showed Thursday.

The data is likely to boost Beijing's argument for its control of the yuan currency but fuel concerns about the country's vast manufacturing sector, which employs millions of workers and has been contracting for several months.

Exports rose 17.1 percent year on year to $169.7 billion, compared with a rise of 24.5 percent in August, the customs agency said in a statement.

Imports expanded 20.9 percent to $155.2 billion in September from a year earlier, compared with an increase of 30.2 percent in August.

"Some impact from weaker global growth was to be expected, and it looks like this is starting to happen," said Brian Jackson, senior strategist at Royal Bank of Canada in Hong Kong.

"We don't expect China's exports to collapse as sharply as they did at the end of 2008, but risks are definitely skewed to a further moderation in external demand in coming months."

Exports to the European Union, China's biggest trade partner, fell to $31.6 billion in September from $34.2 billion in August, customs data showed.

Shipments to the United States were steady at $30.1 billion last month.

"This has been a long time coming," said Alistair Thornton, a Beijing-based analyst at IHS Global Insight.

"We have seen the gradual deterioration in the eurozone and United States markets for a while. It takes time for that to feed into the trade data and it looks like that has started to happen now."

Despite the weak data, the Shanghai Composite Index was up 0.39 percent at 2,429.38 in afternoon trade.

The closely watched trade surplus fell from $17.8 billion in August, marking the second straight month of contraction.

Economists had forecast a $17.25 billion surplus, according to a Dow Jones Newswires survey.

The figure is a perennial point of contention for China's key trade partners, the United States and Europe, who argue the country's currency is grossly undervalued, which gives its exporters an unfair advantage.

Earlier this week the US Senate passed legislation to punish China for its alleged currency manipulation -- a charge Beijing has repeatedly denied.

China has denounced the bill as a "ticking time-bomb" that threatens to blow up trade ties between the economic superpowers, and US House Speaker John Boehner has signalled he would block the bill to prevent a "trade war".

The latest trade data will bolster China's defence of its yuan exchange rate -- which it has repeatedly pledged to make more flexible -- though it is unlikely to silence the critics, analysts said.

China continues to strictly control the value of the yuan despite vowing in June 2010 to let it trade more freely against the dollar. Since then, the currency has appreciated more than seven percent against the greenback.

"Chinese officials will be able to cite today's data as evidence that exporters are already feeling the pinch from the recent appreciation of the yuan in trade-weighted terms," said Jackson.