Investigators have been sent to the southern province of Guangdong to probe a seven-fold surge in precious-metals exports as China intensifies scrutiny of growing irregularities in trade figures.

The team includes staff from the Ministry of Commerce and General Administration of Customs, according to people familiar with the matter who asked not to be identified because the information has not been made public. Shipments of precious metals, including jewelry, rose to about $10.8 billion in September from $1.39 billion a year earlier, according to customs data.

The probe comes after China uncovered almost $10 billion in fraudulent trade nationwide as part of an investigation that began in April last year. The probe found 94.4 billion yuan ($15.2 billion) of loans backed by falsified gold transactions in June.

Hong Kong overtook the US in September as the top destination for mainland shipments, coinciding with renewed appreciation of the renminbi and prompting analysts including those at Everbright Securities Co and Australia & New Zealand Banking Group Ltd to question the data.

"As the renminbi has started to appreciate again and the interest rate differential remains large, round trip trade activity between Hong Kong and the mainland is bound to increase," Liu Ligang, Hong Kong-based head of greater China economics at Australia & New Zealand Banking Group Ltd, said on Thursday. "We have seen some significant discrepancies in Hong Kong's September trade data from the mainland's. Gold, given its nature of small bulk but high value, has been used for such activities in the past."

The central government noticed the rapid increase in trade with Hong Kong of some merchandise and will step up scrutiny, Shen Danyang, spokesman for the Ministry of Commerce, said on Oct 16.

The mainland recorded $1.56 of exports to Hong Kong last month for every $1 in Hong Kong imports, leading to a $13.5 billion difference. The surge included shipments of precious metals, which have been at the center of dodgy invoicing in the past, said Xu Gao, chief economist at Everbright Securities.