Results from a recent survey by the department showed that cucumber, herbs, and salad used in sandwiches at 11 shops in Nha Trang City tested positive for pesticides. At the same time, ham and sauce samples here contained the Salmonella bacteria.

"We carried out both quick testing and lab testing," the department announced on Thursday. Up to 11 of 21 vegetable samples taken for quick testing contained pesticides. Six out of 100 ham, egg sauce, and vegetable samples tested by labs were positive for the Salmonella bacteria."

The department said they decided to survey rising food poisoning cases related to sandwiches reported in Nha Trang and other localities, including Dong Nai, Hoi An, and Lam Dong.

The department said the threat of food poisoning may be higher during the current hot weather. We've asked the Ministry of Agriculture and Rural Development and concerned agencies to intensify inspections on the use of pesticides in vegetables.

In May this year, more than 500 people were hospitalised after eating banh mi sandwiches at the Bang Bakery in the southern province of Dong Nai.

Test results on food samples such as pate, processed pork, sausage and pickled vegetables taken from the bakery revealed the presence of Salmonella bacteria.

Last year, nearly 300 people had food poisoning after eating banh mi sandwiches at the well-known Madam Phuong Bakery shop in Hoi An City.

The Vietnamese banh mi baguette, filled with cold meats, pate, and vegetables, is a popular snack in Vietnam.