Brick-and-mortar US retailers, challenged by growing online sales, now face a new and potentially crushing foe: the coronavirus pandemic.

Major department store chains have closed stores to limit the spread of the disease, cut executive pay, furloughed workers and borrowed heavily.

But it may not be enough to counter the slide in sales as buyers follow government-imposed directives to stay home.

There could be more than 15,000 store closures announced by retailers in 2020 due, in large part, to the coronavirus, Deborah Weinswig, CEO and founder of retail advisory and research firm Coresight Research. told CNBC.

"I think we will see an increase in the number of Chapter 7 [bankruptcy filings]," she added. "Nobody knows how to deal with this."

Clothing retailers have been left with mountains of unsold goods valued at billions of dollars, but the old methods of clearing inventory aren't working because the coronavirus keeps stores closed, The Wall Street Journal reported.

Off-price chains such as T.J. Maxx that usually buy up unsold goods from other retailers also have shut their doors. Liquidators have snapped up merchandise from bankrupt retailers that have halted going-out-of-business sales during the coronavirus outbreak.

The difficult times that major department stores now face also could damage the commercial real estate market. Green Street Advisors, a research firm, said department stores occupy about 30 percent of total mall square footage in the US, and 10 percent of that is leased by J.C. Penney and Sears.

Neiman Marcus, the Dallas, Texas-based luxury retailer, may file for bankruptcy protection this week, Reuters reported. If so, it would be the first major US department store chain to break under the weight of government-imposed stay-at-home orders that have eliminated foot traffic.

The company has temporarily closed its 43 Neiman Marcus stores, about 24 Last Call stores and two Bergdorf Goodman stores in Manhattan. It has furloughed most of its 14,000 employees.

Neiman Marcus is negotiating a loan of hundreds of millions of dollars that would keep the company alive during bankruptcy proceedings.

The company has skipped debt payments. Standard & Poor's said Neiman Marcus owes about $4.8 billion, some of it from the $6 billion buyout in 2013 by Ares Management, a private-equity firm, and the Canada Pension Plan Investment Board.

While the COVID-19 threat is new, online sales aren't, and thanks to lower overhead, online retailers continue to grab market share in the retail sector.

In 2015, e-commerce sales represented 7.4 percent of the market. In 2019, the total had grown to 13.7 percent, or $3.43 trillion of the $25.03 trillion retail market in 2019 – an increase of about 85 percent in four years.

Oberlo, a division of Shopify that helps individuals launch companies in cyberspace, said online sales are expected to increase to 17.5 percent of the market in 2021, a 136 percent increase in six years.

"The department stores, which have been failing slowly for a very long time, really don't get over this," Mark A. Cohen, director of retail studies at the Columbia University Business School, told The New York Times. "The genre is toast, and looking at the other side of this, there are very few who are likely to survive."

Macy's, the largest department store chain in the US with about 775 stores coast to coast, isn't considering bankruptcy but is seeking to raise as much as $5 billion in a debt offering in response to lower sales.

Citing "people familiar with the matter", CNBC said New York-based Macy's plans to use inventory as collateral to raise $3 billion, and put up real estate to raise an additional $1 billion to $2 billion. But the company won't pledge its iconic Herald Square store in Manhattan as part of the deal.

Macy's has closed its stores and lost most of its revenue, but remains active online. The company has drawn on its line of credit, suspended its quarterly dividend, deferred capital spending, cut executive pay and furloughed most sales staff. The company also owns Bluemercury and Bloomingdale's.

Seattle, Washington-based Nortstrom also is seeking to secure additional financing during the economic downturn. According to news reports, it may borrow against some of its real estate.

J.C. Penney faces hefty debt payments and may seek to restructure them in a bankruptcy filing, Reuters reported.

Barney's, an upscale men's store, filed for bankruptcy in 2019 and plans to close this year. The store, founded in 1923, introduced expensive brands, including Armani, Comme des Garcons and Christian Louboutin.

Prior to closing because of the pandemic, department stores attempted to meet the cyberspace challenge competition by offering websites, e-commerce apps and in-store pickups for online purchases.

Some retailers tried one-day sales to move merchandise. Saks Fifth Avenue, an upscale retailer, offered spring dresses with discounts up to 70 percent. Nordstrom cut prices on selected items by as much as 40 percent. J. Crew, which markets to professional men and women, discounted some styles up to 60 percent. Gap, known for its khakis and sweaters, knocked 60 percent off everything.

But now with the pandemic and US unemployment at 22 million, some of the closed retail stores that have struggled to keep their lights on could be faced with keeping them off for good.