WASHINGTON: The Obama administration will seek tougher fuel-economy standards for the nation's heavy trucks and buses as part of its expanded effort to cut down on pollutants blamed for the warming of the planet, officials said Friday.

A proposed regulation would phase in higher efficiency requirements for heavy-duty engines beginning in the model year 2021, eventually leading to a 24-percent improvement in mileage compared to similar vehicles built to meet today's standards, the officials said.

The proposal is the latest in a series of moves intended to slash emissions of greenhouse gases, coming on the heels of previously announced efforts to curb pollution from cars and light trucks, aircraft and power plants. The White House has pledged to reduce overall emissions in the United States by at least 26 percent by the year 2030, compared to 2005 levels.

Unlike some of the earlier steps, the proposed rules are backed by many in the trucking industry as a way to lower fuel costs borne by drivers and, ultimately by consumers. Administration officials said the measures, if finalized, could reduce spending on gasoline and diesel by an estimated $170 billion.

"Once upon a time, to be pro-environment, you had to be anti-big-vehicles. This rule will change that," said Transportation Secretary Anthony Foxx, who announced the proposal jointly with officials of the Environmental Protection Agency. "In fact, these efficiency standards are good for the environment, and the economy."

The proposed regulation affects a class of vehicles that includes the largest tractor-trailers as well as buses, large pick-up trucks and vans. These heavy- and medium-duty vehicles represent only about 5 percent of total highway traffic but account for 20 percent of fuel consumption and carbon emissions.

The EPA said the proposals, if finalized, would reduce oil consumption by nearly 2 billion barrels and lower carbon emissions by 1 billion metric tons. The new standards would raise vehicle prices by as much $14,000 for a large truck, according to agency projections, but the higher sticker price would be offset by fuel savings in about two years, EPA officials said.

The measures also include tougher standards for trailers, requiring manufacturers to use light-weight materials and more aerodynamic designs to improve fuel economy.

The proposals overall will "save consumers, businesses and truck owners money," EPA Administrator Gina McCarthy said, "and at the same time spur technology innovation and job-growth, while protecting Ameri-cans' health and our environment over the long haul."

The reaction among truckers' groups was mostly positive, though some expressed concerns about whether the technology needed for improved fuel economy was fully in place.

"Fuel is an enormous expense for our industry, and carbon emissions carry an enormous cost for our planet," said Bill Graves, president of the American Trucking Associations.

But some truck dealers warned that the higher vehicle costs would hurt truckers and the economy.

"Adding $14,000 to the cost of a new truck through mandates based on potentially untested technology is a great risk to a still fragile economy," the National Automobile Dealers Association and American Truck Dealers said in a joint statement.

Environmental groups and local regulatory agencies welcomed the tighter fuel requirements in a week in which Pope Francis sought to frame the fight against climate change as a moral challenge.

"The standards will sharply reduce climate pollution from the transportation sector and will reduce America's reliance on imported," said Fred Krup, president of the Environmental Defense Fund, a nonprofit advocacy group.

William Becker, executive director of the National Association of Clean Air Agencies, which represents air pollution control agencies in 41 states, said the reductions would "benefit, literally, every community across the nation."

"The beauty of the proposal is that the cost of the necessary improvements, which are generally off-the-shelf technologies, will be paid for by the savings associated with the increased fuel efficiency — about one-third better than today — in a matter of a couple of years," he said.

The Obama administration has touted gains in automobile fuel efficiency as one of its signature environmental achievements. The White House had already adopted tougher standards for light vehicles and trucks during President Barack Obama's first term.

A poll released Friday by the Consumer Federation of America suggested that most Americans supported higher fuel-economy standards for trucks, even if it meant paying more for transported goods. One-thousand poll respondents surveyed in May voiced support for a hypothetical new rule by a margin of 71 percent to 24 percent, the group said.