Vietnamese police are investigating an international diamond ring accused of smuggling 28,000 gems worth nearly $11 million, with the country’s largest jeweller caught up in the probe.

The former head of Phu Nhuan Jewelry’s (PNJ) gem certification subsidiary was arrested earlier this month along with more than 30 others accused of involvement in a syndicate operating over several years.

Police have raided more than 20 establishments, including jewellery stores and homes of the accused, mostly in Ho Chi Minh City, the public security ministry said in a statement late on Tuesday.

State media reported on Wednesday that a number of establishments linked to the investigation have suddenly closed their doors.

Dang Ngoc Thao, the former director of PNJ Laboratory, is accused of falsely certifying gems sourced in India for resale in Vietnam, removing laser inscriptions and replacing them with the company’s own.

Police have seized 1,239 diamonds in the investigation so far, along with jewellery “for which no invoices or documents proving their origin could be produced”, the ministry said.

The suspects “employed highly sophisticated criminal methods” to source diamonds from India and organise their “illegal transport” via Hong Kong into Vietnam, it added.

In a statement, PNJ said it “acknowledged” the investigation into its former employee but claimed the “matter involves individual legal liability”.

It said diamond products certified by its subsidiary are “fully traceable and of guaranteed quality”.

Shares in PNJ, Vietnam’s largest listed jeweller, were down more than 25 per cent since the first arrests were made in early July.