Nga of Hanoi’s Cau Giay District said on Facebook she recently sold 30 boxes (of 50 masks) at VND120,000 ($5.2) each, almost twice the regular price of VND65,000.

She blamed the supplier for increasing prices, saying she next plans to sell a box at VND160,000 ($6.9) in the next two days.

"I don’t get much profit from this, just trying to help people combat the pandemic. Masks are rare now, and their prices are going up by the hour."

Trang of Ho Chi Minh City’s Thu Duc District too hiked her prices to VND120,000 ($5.2), saying, "I bought the masks for myself, and sell to those who need them."

Those who do not buy soon might have to pay even more in the coming days, she added.

Trung of Binh Tan District said he sold 50 boxes of masks in an hour for VND90,000 ($3.9) but "lost" money. "Buy them quick, as their prices will go up tomorrow."

Many online sellers hiked prices by 40-100 percent over the weekend after 15 people were diagnosed with Covid-19 in the central region. Da Nang City authorities have imposed social distancing measures starting Monday.

But major retailers in Hanoi and HCMC said they have enough supply of masks and would continue to sell them at normal prices of around VND65,000 ($2.8).

A spokesperson for VinCommerce, which operates supermarket and convenience store chains VinMart and VinMart+, said 2.5 million boxes of masks and three million hand sanitizers are in stock and sold at normal prices.

"We guarantee there will be enough masks and hand sanitizers for people to combat the pandemic."

Ichiban Market, a supermarket in Ho Chi Minh City, also said there is no scarcity of masks.

Manufacturers too assured that there are enough stocks. Le Tien Truong, CEO of textile company Vinatex, said his company could produce 100 million masks a month, and that prices would not change.

The Ministry of Industry and Trade has said that the 50 biggest garment producers in the country could produce 200 million masks a month, enough to meet domestic demand.

Market authorities have begun to monitor the prices of masks and other pandemic-related goods to punish speculators and gougers.

In Da Nang, where 14 cases of Covid-19 have been confirmed since Saturday, authorities checked nearly 600 medical equipment outlets and found no unusual rise in prices despite the sharp rise in demand.