The Toyota Prius hybrid, once viewed as a science experiment for environmentalists, has become a mainstream vehicle in the United States, with the company selling more than a million of the cars nationwide.

Toyota announced Wednesday that it had passed the million sales milestone, helped by brisk sales recently as U.S. gasoline prices continued to rise toward $4 per gallon ($1.05 a liter).

The Prius, which is powered by both a gasoline engine and an electric motor, gets an estimated 51 miles per gallon (22

kilometers per liter) in the city and 48 (20) on the highway. Aside from electric vehicles, it is the most fuel-efficient car in the U.S., according to the Environmental Protection Agency.

“Prius paved the way for hybrids,” Bob Carter, Toyota Division group vice president and general manager in the U.S., said in a statement.

Toyota began selling the Prius 11 years ago when gas prices were low. At first it was a car for the environmentally conscious, but as prices spiked during the decade, its popularity increased. In the first quarter of this year, Toyota already has sold nearly 43,000, which is 52 percent more than at the same time last year, according to Autodata Corp. In 2010, the company sold more than 140,000 Priuses.

Prius sales worldwide topped 2 million last year, and Toyota said its total global hybrid vehicle sales passed the 3 million mark in last month.

“Prius has become synonymous with the word hybrid, and as we see fuel prices starting to rise again, it has accounted for more than 60 percent of hybrid passenger car sales so far this year,” Carter said.

Some Toyota dealers have been reporting that Priuses are in now in short supply because of high demand and factory outages in Japan due to the March 11 earthquake. The quake and tsunami forced Toyota to stop production at most Japanese factories. But the company restarted two plants that make Prius and Lexus models on March 28.

Moody's Investors Service warned Wednesday it may downgrade its credit rating for Toyota Motor Corp. due to the financial fallout from suspended car production from the quake and tsunami. - Sapa-AP