Fitch Ratings cut Delta Air Lines Inc. to junk, and downgraded American Airlines Group Inc. and Southwest Airlines Co., as part of a portfolio review of most airlines in North America, amid depressed levels of airline traffic due to the coronavirus pandemic.

The review suggests that the coronavirus pandemic will reverse credit improvements for North American airlines, increased debt levels and reduce liquidity.

The outbreak will likely cause "substantial" cash outflows in the near term, Fitch said, with a slower rebound in air traffic than previously predicted becoming increasingly likely.

"A longer period of depressed airline traffic, combined with debt raised to manage liquidity, will cause all carriers to operate with weakened credit metrics and reduced financial flexibility at least through 2021," Fitch forecasts.

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The rating agency downgraded Delta to BB+ from BBB- with a negative outlook, reflecting concerns that debt raised will keep its credit metrics below those supportive of investment-grade ratings at least through 2021 and likely into 2022.

Fitch forecasts leverage to remain above 3x through 2022 following a spike this year, as Delta is expected to borrow more than $8 billion in 2020.

American Airlines was cut by one notch to B from B+, with its ratings placed on rating watch negative. Fitch expects American Airlines' leverage to possibly remain above 6x through 2022, noting that the company's credit metrics were already under pressure prior to the pandemic.

Fitch previously expected metrics to improve over the next one to two years, but that improvement has now been pushed further into the future.

Meanwhile, Fitch downgraded Southwest Airlines to BBB+ from A- with a negative outlook, reflecting additional borrowing needs during the pandemic. However, the agency believes that the airline is the best-positioned U.S. carrier to survive the crisis due to a "strong balance sheet, unencumbered asset base and domestic focus."

Fitch also downgraded United Airlines Holdings Inc., Alaska Air Group Inc., JetBlue Airways Corp., Spirit Airlines Inc., Hawaiian Holdings Inc. and affirmed Air Canada as part of the review.