The Environmental Protection Agency on Wednesday called for regulating U.S. aircraft emissions, expanding the government's effort to crack down on industries officials say are contributing to global warming. 

The EPA issued what's known as a preliminary finding of endangerment. The agency declared the emissions are harmful to human health and contribute to climate change -- a declaration that lays the groundwork for the government to eventually regulate the airline industry. 

Critics, though, have warned that the looming rules could eventually lead to increased ticket prices -- and more crowded flights -- as airlines try to comply with new efficiency standards. 

“The sky is the limit when it comes to how much of the U.S. economy the EPA wants to control,” Rep. Lamar Smith, R-Texas, said in a written statement following the EPA announcement.

Wednesday’s finding coincides with a multi-year push by the International Civil Aviation Organization, a branch of the United Nations, to develop global aircraft emissions standards. Final agreement on the ICAO standards, a U.N. agency, is expected in February 2016.  The standards themselves aren’t expected to go into effect until 2020 or even as late as 2025, according to some environmentalists following the matter.

A final U.S. decision on adopting the international standards is likely to be left to the next presidential administration. 

Smith, chairman of the House Science, Space and Technology Committee, said the regulations “would increase the price of airfare for Americans and harm our domestic carriers.” He added that the EPA finding is the “next leg of a nonstop journey by the EPA to control how Americans live, work and travel.” 

The industry, though, praised the EPA for planning to go through the international process. Nancy Young, with Airlines for America, said it is "critical" that the emissions standards be agreed upon globally. 

"U.S. airlines are green and we are getting even greener," she said in a statement. 

The U.S. regulations would apply only to large planes like airliners and cargo jets and turboprop aircraft, and not to smaller jet aircraft, piston-engine planes, helicopters or military aircraft.

The ICAO standards are not expected to apply to airliners in service today or those that might be purchased before the effective date, said Vera Pardee, an attorney with the Center for Biological Diversity. The center is one of several environmental groups that sued the EPA to force the agency to issue its finding that emissions endanger public health.

Airlines typically fly planes for 20 years or more before replacing them. That means it's likely to be decades before planes that meet the anticipated global standards are in widespread use.

Airline emissions account for about 2 percent of total annual global greenhouse gas emissions. That sounds small, but it's nearly as much as the emissions produced by Germany, the sixth-greatest greenhouse gas producing country, according to a study released last year by The International Council on Clean Transportation, an environmental group with offices in the U.S. and Germany.

Aircraft manufacturers have already made significant strides in increasing fuel efficiency. Since the early years of the jet age in the 1960s, the fuel efficiency of airliners has increased 70 percent, according to Boeing. There's plenty of incentive to be as efficient as possible: Fuel typically vies with labor as airlines' greatest expense.

The U.S. airline industry has set a target of an average annual improvement in fuel efficiency of 1.5 percent, and so far has been successful in meeting that goal, said Young. 

Alaska, Frontier and Spirit airlines were tied for most fuel-efficient U.S. airlines, the study found. The least fuel efficient was American, which operates a fleet of MD-80 airliners, an older design that is being phased out.

Changes in the operating strategies of airlines in recent years have also contributed to greater efficiency. Airlines are packing more people into fewer flights.

However, global aviation emissions are rising because there is more air travel overall. U.S. airlines, which include several of the world's largest carriers, account for about 29 percent of global airline carbon emissions if both domestic and international flights are included.

The world's two largest aircraft makers have recently introduced into service more fuel-efficient planes designed for long-distance international routes — the Boeing 787 and the Airbus A350.

Airlines, aircraft makers and the Federal Aviation Administration have also been working with biofuels companies to develop alternatives to jet fuel that could potentially reduce the aviation's industry's vulnerability to the ups and downs of oil supplies and prices, as well as reduce carbon emissions.

"We're not dragging our feet," said Tim Neale, a spokesman for Boeing. "We're hard at work on lighter airplanes, and GE is hard at work on more efficient engines. And we're working a lot of these operational issues with the carriers so they operate the planes more efficiently."

Boeing and airline industry officials say they support ICAO's effort to develop a single global standard, since airlines fly globally. But Pardee said environmentalists hope that if the ICAO standard turns out to be weak, the EPA will move forward with stronger standards for U.S. airlines.

The Associated Press contributed to this report.