DETROIT — Ford Motor Co., the second-largest U.S. automaker, began taking orders yesterday for a hybrid version of its Mercury Mariner sport utility vehicle a year earlier than planned in response to increased interest.

Lincoln Mercury division spokeswoman Sara Tatchio said Ford already had received 27 orders for the Mercury Mariner Hybrid by early afternoon. The SUV will be sold almost exclusively online. Customers can order the vehicles through Mercury's Web site and pick them up from a local dealer.

The San Francisco-based Sierra Club said it will tell its members about the Mariner Hybrid and offer test drives at its annual summit in September. It's a change of pace for the environmental group, which ran ads two years ago criticizing Ford's environmental record. At the time, the Sierra Club said Ford's 95-year-old Model T was more fuel efficient than the Ford Explorer SUV.

"For years, the Sierra Club has pressured Ford to make more fuel-efficient cars and trucks," said Dan Becker, director of the Sierra Club's global warming program in Washington. "They are now beginning to do that, and we want to help them succeed."

The Mariner Hybrid's fuel economy is nearly 50 percent higher than a conventional Mariner, Ford said. The Mariner Hybrid gets an estimated 33 miles per gallon in the city and 29 mpg on the highway.

Not all environmental groups are satisfied, however. The San Francisco-based Rainforest Action Network said it will run a newspaper ad in a few weeks targeting Ford for having the least fuel-efficient fleet of all the major automakers.

"While it's encouraging that Ford is putting a second hybrid on the market, the production levels are so low that it will have no measurable impact on Ford's bottom-of-the-barrel fleetwide fuel efficiency or off-the-charts greenhouse gas emissions," Rainforest Action Network executive director Michael Brune said.

Ford plans to produce 2,000 Mariner Hybrids at its Kansas City, Mo., plant for the 2006 model year. The company believes volume will eventually grow to 4,000 vehicles annually. Ford sold more than 3 million vehicles in the United States last year.

Ford's director of hybrid programs, Mary Ann Wright, said Ford is committed to reducing emissions in all its vehicles. The Mariner is the second of five hybrids the company plans to introduce in the next few years, she added.

The automaker introduced the Ford Escape Hybrid last fall and sold 2,566 vehicles in 2004, or about 3 percent of the total hybrid market, according to R.L. Polk & Co., a Southfield-based automotive data firm. Japanese brands accounted for more than 96 percent of the hybrid vehicles registered.

The Escape Hybrid was the first hybrid sport utility vehicle on the market and the first hybrid produced by a U.S. automaker. Since then, Toyota Motor Corp. has released hybrid versions of two SUVs: the Toyota Highlander and the Lexus RX330.

The 2006 Mercury Mariner Hybrid SUV starts at $29,840, or $4,190 more than a Mariner with a traditional engine and a luxury trim package, Tatchio said. The Mariner Hybrid is $7,800 more than a Mariner at the standard trim level, but since the Mariner Hybrid has luxury features, that's not an equal comparison.

The price difference is similar for a Ford Escape and a Ford Escape Hybrid, Tatchio said. Hybrid vehicles typically cost $4,000 to $9,000 more than their traditional counterparts, according to Polk data.

"The price is not yet at a stage where it is coming down. It is extremely new technology," Tatchio said.

Ford shares were up 29 cents to close at $10.71 on the New York Stock Exchange.