"Black Friday" has kicked off in the United States with companies offering a new batch of "doorbuster" sales and promotions to early holiday shoppers, but online data show that consumers have been spending big for weeks.

The day after the US Thanksgiving celebration is the traditional start to the holiday shopping season, and normally sees Americans line up outside stores before they open to clinch deals on popular items.

After the pandemic subdued last year's spending, anticipation is high this year since millions of people have returned to jobs lost to business restrictions while vaccines have reduced the threat from Covid-19.

Before retailers opened their doors early Friday morning, e-commerce shoppers in the United States had already spent $76 billion since early November, up more than 20 percent from the year-ago period, according to data from software company Adobe.

The jump has added to companies' optimism about the season, suggesting some shoppers heeded calls from chains to purchase items early this season after port backlogs and other supply chain problems prompted worries about shortages of popular goods.

Toys led the buying spree, with Adobe pointing to actions by "anxious parents increasingly aware of supply chain challenges."

The National Retail Federation projects overall spending could rise as much as 10.5 percent to $859 billion.

Nonetheless, out-of-stock listings online are up 261 percent compared with two years ago, according to Adobe.

Retailers and market watchers are broadly optimistic about the holiday shopping season in light of low unemployment and relatively strong household finances due in part to pandemic stimulus bills enacted by the government.

Countering those positive trends are spiking consumer prices that have affected household staples such as food and fuel, and lingering supply chain problems that are leading to frequent "out of stock" notices on websites and in shopping aisles.

"Black Friday" shoppers wait in line for a Best Buy store to open on November 26, 2021 in Westminster, Colorado
"Black Friday" shoppers wait in line for a Best Buy store to open on November 26, 2021 in Westminster, Colorado GETTY IMAGES NORTH AMERICA via AFP / Michael Ciaglo

"Pandemic-related supply chain disruptions have caused shortages of merchandise and most of this year's inflationary pressure," said NRF Chief Economist Jack Kleinhenz.

"With the prospect of consumers seeking to shop early, inventories may be pulled down sooner and shortages may develop in the later weeks of the shopping season," he said, but predicted "a stellar holiday sales season" if stores keep shelves stocked.

Another wildcard has been Covid-19. On Friday, global stock markets tumbled on worries that the latest strain of the virus found in South Africa could derail the global recovery.

To navigate these difficult conditions, retailers have taken extraordinary steps such as importing and storing items earlier than usual, ordering shipments by air freight and in some cases even chartering their own vessels.

Retailers have also been paring back some promotions in light of elevated costs.

Chains have had mixed results when it comes to ensuring supply, with big box stores such as Walmart and Target building inventories, but other outlets such as Gap and Nordstrom reporting lost sales due to shortages in their stock.

An emerging worry in the industry is that retailers will be stuck with goods originally intended for the holidays but that don't arrive until January.

Macy's is generally canceling orders for items with a Christmas motif, but plans to keep other items if they are cold-weather-oriented and could sell later in the winter, executives said earlier this month.

Gap Chief Financial Officer Katrina O'Connell said the apparel chain was planning to hold some items for next winter.

"If we think items are going to be too late for the holiday season, we won't put them in stores or online and have them generate markdowns," she said earlier this week on a conference call with Wall Street analysts. "We'll hold them for next year."

Gap has been one of the companies hardest hit by supply chain problems due to lengthy outages in Vietnam caused by the country's Covid-19 restrictions, which contributed to a loss of some $300 million in sales in the most recent quarter.