A woman in a protective mask shops as a line of shoppers stretch to the back of the store waiting to check-out at the Market Basket in Chelsea, MA on March 13, 2020.

As U.S. grocery stores try to keep shelves stocked during the coronavirus outbreak, some major chains have begun to limit purchases of food items and ramp up hiring.

Texas grocery chain H-E-B and Wegmans are limiting purchases of more than a dozen popular food items, including bread, canned soup, frozen vegetables and boxed pasta. Limits vary by item. At H-E-B and Wegmans, for example, eggs and milk are limited to two units per customer.

Over the weekend, the companies' lists of items with purchasing limits has continued to grow.

Kroger said it's ramping up hiring across the country to keep up with the spike in demand.

Food purchase limits and increased hiring are grocers' latest response to a surge in grocery shopping in the U.S. As the number of confirmed coronavirus cases has risen and health officials urge social distancing, Americans have rushed to stores to stock up on canned goods, hand sanitizer, toilet paper and other items. The panic buying has challenged grocery stores' supply chains, as shelves are cleared more quickly than usual and workers must race to keep up.

"Grocery stores become the center of the community," said Doug Baker, vice president of industry relations at food retail trade group FMI. "It's the lifeline during a time of disaster. They are on the front lines."

Baker said the industry group — which represents grocery stores, wholesalers and product manufacturers — is encouraging its members to limit purchases of food items to make sure all customers can get what they need. He said he expects more limits on food purchases in the weeks ahead.

He said such limits help prevent hoarding, which can lead to price-gouging by third-party vendors or shortages that can hurt people who are sick or vulnerable. He said they also can create "some calmness and a game plan" as grocers cope with heavily trafficked stores.

"In order for us to get through this, it can't be me against you," he said. "We're really going to have to come together as a society."

On Thursday, H-E-B began limiting some food items at stores. It added more items Friday and is continuing to update the list as inventories change, said Mabrie Jackson, director of public affairs for H-E-B and Central Market.

On its website, the grocer said that it is "in a good in-stock position on many of the items our customers need," but said limits "help protect the supply chain in Texas." If an item is low or unavailable, it encouraged customers to return later as the grocer continues to restock.

Other grocery stores are expanding their workforce and supporting employees during an especially busy time.

Kroger is advertising immediate openings for employees on its website and in customer emails. The national grocery company, which owns chains including Fred Meyer, is filling jobs at stores, distribution centers and manufacturing plants, Kroger spokeswoman Kristal Howard said.

Walmart, the country's largest grocer, has given its managers the option of limiting purchases, and has announced reduced hours for its stores that are open around the clock. This will allow staff to restock shelves and clean stores more efficiently.

Walmart thanked associates in a video message recorded Friday. In the video, Walmart U.S. CEO John Furner said the company's stores, website, distribution centers and fulfillment centers have all been busier than usual. He said the retailer has "a lot of categories that have seen extraordinary growth levels the last few weeks," including over-the-counter pharmaceuticals and food.

"I've seen photos of big crowds in stores," he said in the video. "It's been managed well, and I think every associate should be proud of what you're doing and know that what you're doing right now matters to people."

Walmart was among the retailers who met with President Donald Trump on Friday at the White House. Walmart, Target, CVS and Walgreens said they will offer drive-up testing for the coronavirus in their parking lots. The retailers said they're working with the Trump administration on timing and locations.

At a press conference in the Rose Garden Friday, Walmart CEO Doug McMillon acknowledged the challenge of replenishing some items, including cleaning products and paper goods. "Hand sanitizer is going to be very difficult to have 100% in stock on for some time," he said.

Wegmans thanked customers for their support in a message on its website and asked for patience as they do weekend shopping.

"We recognize you may not find everything you expect at the moment and we appreciate your understanding," the company said. "If we can all please consider prioritizing our most immediate needs, it will allow us to best serve the needs of our entire community."