New York (CNN Business) Some of the largest grocery chains in the United States, including Kroger, H-E-B and Walmart, are changing store hours during the coronavirus outbreak. They say the reduced hours will give them more time to clean stores, restock and keep workers healthy.

, the largest US supermarket chain, said it was adjusting store operating hours in some areas "based on local circumstances."

"We will continue to make decisions that allow us to operate clean, open and stocked stores to serve our customers and support our associates," the company said in a statement on its website.

Kroger said two employees -- one at a King Soopers store in Colorado and another at a Fred Meyer's store in Washington state -- have tested positive for the novel coronavirus. "Upon learning of these cases, we partnered with state and local health experts, followed all sanitation and cleaning procedures, communicated with and supported our store teams, and with the support of the state governments, the stores remain open," a statement said.

Earlier this month, Kroger said it was capping individual purchases of "sanitization, cold and flu-related products," to deal with panic buying

Grocery chain Publix, which has more than 1200 stores across Florida and southern states, said on Saturday it will be closing at 8 pm ET, or two hours early, to give employees time to restock produce and sanitize shelves.

"Publix will continue to focus on keeping our associates healthy — and our stores open and stocked — to serve and support all our communities," Todd Jones, Publix CEO, said in a statement on the chain's website

Jones noted that food demonstrations were suspended until further notice and the company has set purchase limits "on some key items to allow more customers to get what they need."

Publix's pharmacies will also close at 8 pm ET until further notice.

On Facebook, many customers thanked Publix for its work, while others noted that the stores were still running out of toilet paper and hand sanitizer.

The modified hours come as US and local officials take extra steps to stop the spread of the virus in the United States. President Donald Trump on Friday declared a national emergency to free up $50 billion in federal aid. The virus' spread around the world has rattled markets and disrupted industries including travel, hospitality and manufacturing.

Trump on Sunday spoke by telephone with more than two dozen grocery store and supply chain executives from across the country, according to the White House.

"Supply chains in the United States are strong, and it is unnecessary for the American public to hoard daily essentials," according to a readout of the conference call provided by White House Deputy Press Secretary Judd Deere.

The telephone call comes a day after several other grocery chains said they were modifying store hours to allow for restocking and cleaning.

Texas chain H-E-B announced Saturday it would modify its store hours to 8 am CT to 8 pm CT to allow its employees "to work diligently overnight to fully prepare the store for customers during the day."

It tweeted that it would return to normal hours and services "as soon as we can" and added the hashtag #SlowTheSpreadTexas.

The Giant Company, a chain located in Pennsylvania with nearly 200 stores across the mid-Atlantic, said its 24-hour locations would start closing at midnight and reopening at 6 am ET the following day. It cited the same reasons as Publix in a press release: "The decision comes as part of the company's continued response to the COVID-19 pandemic and will allow store team members additional time to sanitize and restock shelves as daily deliveries arrive."

Giant's 24-hours stores are mostly in Pennsylvania, with one location in Maryland. Pharmacies located inside those stores will continue normal hours.

"We are currently experiencing shortages and out of stocks on many household staples, including disinfecting and cleaning products. We are focused on getting back in stock as soon as possible," said Ashley Flowers, a public relations manager at Giant, in a statement to CNN Business.

Giant has not set a limit on how much of an item each person can purchase. It's also cleaning shopping carts, baskets, checkout kiosks and more. Like Publix, Giant has stopped in-store sampling events, according to Flowers.

Harris Teeter, a large supermarket chain serving the southeast tweeted Saturday that it will close its stores at 9 pm ET each night in order to focus on cleaning and replenishing and for the health of their employees, starting Sunday night.

Walmart WMT , the largest retailer in America, said Saturday that it will modify its store hours in response to the pandemic.

Beginning Sunday, all of Walmart's more than 4,700 US stores will be open from 6 am to 11 pm until further notice. The company said the shortened hours will help employees restock shelves overnight and clean stores.

Trader Joe's said that beginning Monday and until further nontice, all of its stores will be open from 9 am to 7 pm "to support our Crew Members in taking care of one another and our customers."