UnitedHealth Group Inc., the Minnetonka health insurance giant, has approached rival Aetna Inc. about a takeover deal that would likely be valued at more than $40 billion, the Wall Street Journal reported Monday night.

The approach came in the form of a letter, the Journal said, citing an anonymous source familiar with the matter, and it wasn't known what Aetna's response was, if any.

The report is yet another drumbeat for consolidation in the health insurance industry as observers speculate that the industry's Big Five — UnitedHealth, Anthem, Humana, Cigna and Aetna — are about to become the Big Three.

Humana Inc. is already exploring a sale after being approached by Cigna about a potential deal, Bloomberg News reported last month, citing a person with knowledge of the matter. Anthem Inc. has explored a takeovers of smaller rivals Cigna and Humana, Bloomberg reported, citing a person with knowledge of the matter. In a third possible combination, Humana has also drawn interest from Aetna, another person said.

In trading Monday, Cigna rose as much as 19 percent and Aetna climbed as much as 7.2 percent. UnitedHealth shares rose $1.33, or 1.1 percent, to close Monday at $118.98.

Humana offers Cigna a way to gain the lead in the market for Medicare Advantage policies, the health insurer-run version of the U.S. program to cover the elderly and disabled.

Jon Sandberg, a spokesman for Cigna, and Kristin Binns, a spokeswoman for Anthem, declined to comment to Bloomberg News. Tyler Mason at UnitedHealth and Cynthia Michener at Aetna also declined to comment.

Health insurers in the United States have been trading near all-time highs after enjoying an influx of new business from the Patient Protection and Affordable Care Act, which brought previously uninsured people into the market for the first time. At the same time, the law put pressure on profit margins by imposing new fees on the companies and mandating that they spend at least 80 percent to 85 percent of premiums on medical claims.

With the initial surge of Affordable Care Act enrollment now leveling off, the companies are looking at mergers to cut costs and keep profits expanding.

Cigna rebuffed the latest offer from Anthem for about $175 a share, the Wall Street Journal said, citing people familiar with the discussions.

Buying Cigna would help Anthem expand in employer-provided health insurance and in overseas markets. Cigna provides insurance to employees of U.S. companies working internationally and also sells products in countries such as South Korea. Anthem sells coverage under the BlueCross BlueShield brand in states including California, Georgia, New York and Connecticut.

"Strategically, the transaction would improve the competitive position of both concerns," Chris Rigg, an analyst at Susquehanna Financial Group, said in a research note.

Amid speculation of consolidation in the industry, analysts have been working to figure out how the deal frenzy will shake out. If Anthem targets Cigna, Aetna is likely to pursue Humana, Rigg said.

"Alternatively, if Cigna really doesn't want to sell, then the obvious defense is to buy Humana at any cost," he said. "Crazy times."

Bloomberg News contributed to this report.