Vietnam is struggling to curb fraud in exports destined for America, custom officials said Thursday, as manufacturers seek to dodge punishing tariffs due to the US-China trade spat by illegally using "Made in Vietnam" labels.

Exporters have started shifting production from China to Vietnam to avoid steep tariffs imposed by US President Donald Trump on hundreds of billions of dollars worth of Chinese goods.

The move has also prompted concerns on a rise in manufacturers illegally claiming their products are from Vietnam when they in fact originate in China -- and authorities have vowed to crack down with more stringent inspections and punishments.

Custom officials in Hanoi said Thursday they currently lack the resources to properly tackle the issue, and only five percent of all import-export declaration forms are verified.

"We can't check all the goods. Only when we identify the risks or signs of violations will the customs authorities check," said Au Anh Tuan, director of customs control and supervision at Vietnam Customs.

On the sidelines of a trade origin fraud workshop, he told AFP there have been cases where "Made in Vietnam" labels were sewn in after goods have cleared customs, making it more difficult to flag as a potential risk.

"There is not enough staff from market regulation authorities or inspectors to check that," Tuan said.

The US has seen a "tremendous amount" of fake Made in-Vietnam goods recently, said Tom Jesukiewicz, a field director at the US Customs and Borders Protection, declining to name specific figures.

However, he singled out steel products and plywood as problem areas.

"We saw an extreme pattern shift immediately," Jesukiewicz told AFP.

Any sector with more than 15 percent growth is considered "high risk", said Mai Xuan Thanh, deputy director general of Vietnam Customs, adding that authorities will continue reviewing businesses shipping to the US.

According to Vietnam's Ministry of Industry and Trade, some 15 commodities -- including textiles, footwear, electrical components, iron, steel and wood products -- have been identified as fraud risk due to a sudden surge of imports from China and exports to markets like the US.

The US and China have slapped steep tariffs on hundreds of billions of dollars in two-way trade, and another round of US duties are set to hit on December 15 on $160 billion in Chinese goods.

Vietnam has long been a manufacturing hub for cheaply-made goods, ranging from Adidas sneakers and H&M dresses to Samsung smart phones and intel processors.

In the first nine months of this year, US imports from Vietnam reached nearly $44.9 billion, up more than 28 percent from the same period last year.

The rapid increase has not gone unnoticed by Washington, as Trump in June singled Vietnam out as "the single worst abuser of everybody" and threatened to slap levies on them too.