Inc.’s Prime Now and Amazon Fresh delivery services have been overwhelmed by demand, a sign that virus-spooked shoppers are turning to the world’s largest online retailer to avoid going to brick-and-mortar stores.

Amazon on Monday warned customers that both services would have limited availability, meaning orders are being delivered more slowly than usual. The company hasn’t reduced the number of people or trucks dedicated to either service, but it has seen a surge in demand that’s straining its delivery capacity, according to a person familiar with the matter, who requested anonymity because they aren’t authorized to speak publicly about the issue.

Prime Now is Amazon’s quick delivery service available to Prime subscribers who pay monthly or yearly dues in exchange for delivery discounts and other perks. It offers delivery of a limited selection of products similar to what is found in a convenience store in as little as an hour. Amazon Fresh is a grocery delivery service available to Prime members and part of Amazon’s foray into the $840 billion grocery business led by rival Walmart Inc.

One Amazon delivery driver who spoke on condition of anonymity said the company has seen a surge in demand for bottled water. Amazon Flex drivers, who make Amazon deliveries in their own vehicles, don’t like delivering water since sometimes it adds hundreds of pounds to their vehicles and is much harder to deliver than a small package.