Chinese exports rose 9.9 percent in September year-on-year to a record monthly high, the government said Saturday, but analysts warned the performance was unsustainable given the weak global outlook.

The national customs bureau also said the trade surplus of the world's second-biggest economy, a source of friction with China's trading partners, widened to $27.7 billion for the month, up from $26.7 billion in August.

Global concern has mounted over a weakening trend in the Chinese economy, an important driver of global growth, and in exports in particular as the broader global slowdown and European debt crisis have impacted shipments from China.

But exports, the key indicator of the health of China's vital manufacturing sector, climbed 9.9 percent in September, the General Administration of Customs said. Imports rose 2.4 percent.

Both figures beat the expectations of analysts polled by Dow Jones Newswires, which had predicted a median forecast of 5.0 percent growth for exports and 2.0 percent for imports.

The September data will come as good news in a year when export growth rates have softened significantly from the strong expansions of previous years.

However, barring a significant improvement in overall trade conditions, the performance was unlikely to be repeated, said Alistair Thornton, China economist with IHS global Insight.

"There is no evidence to suggest that global trade activity is picking up, so there is nothing to support stronger Chinese exports," he said.

Export growth was likely to fall back to around five percent in coming months and imports would slip to near-zero growth, he said.

Citigroup economist Ding Shuang said the trade data is "a positive sign for the Chinese economy", according to Dow Jones Newswires.

But he agreed that it looked unsustainable and that a government target of 10 percent growth in overall trade would likely not be met.

China's gross domestic product grew 7.6 percent in the second quarter of this year, the slowest rate since 2009.

Chinese authorities have been aiming for 7.5 economic percent growth for 2012 -- far less than the 9.3 percent achieved in 2011 and 10.4 percent in 2010.

The world's largest exporter has been pulled down by weakness in overseas economies including debt-saddled Europe, a major trading partner, as well as a sluggish property market and softening consumer spending.

Overall trade for the first nine months of the year was $2.84 trillion, up 6.2 percent from the same period in 2011, the customs bureau said, off the pace of the 10 percent target.

Overall trade with Europe was down 2.7 percent to $411 billion for the first nine months of the year, but trade with the United States grew 9.1 percent to $355 billion.

Following the trade data, markets will turn their eyes toward the release of third quarter GDP data on October 18, and whether the Chinese economy deteriorated further.

Thornton said China was unlikely to come out soon with a significant economic stimulus package as leaders were gearing up for an important Communist Party meeting next month during which a new set of top leaders will be confirmed.

Major policy initiatives are typically put on hold in the run-up to such gatherings.

Trade with Japan slumped 1.8 percent to $248 billion for the year's first three quarters, according to the customs data.

A bitter dispute that erupted in recent months between China and Japan over an island group has stoked anti-Japan sentiment in China, impacting sales of Japanese products in September.

However, the customs bureau made no link between the row and the trade performance.