The U.S. Environmental Protection Agency is aiming to publish by the end of April its "tailoring rule" outlining which power plants and other sources of greenhouse-gas emissions will be regulated, an EPA official said Tuesday.

Emitters such as refineries, the oil industry, steel smelters, cement and brick kilns, and chemical plants are carefully watching how and when the EPA will regulate greenhouse-gas emissions.

"We expect the rule to be done very shortly, hopefully by the end of the month," Assistant Administrator Regina McCarthy said, but added that it may be pushed into May given the complexity of the rule.

Industry officials say uncertainty and concerns about the new regulations--how stringent and flexible they will be--are curbing energy development and clouding future investments in emitting sectors.

McCarthy said the rule "will take some of the fear away about what permitting requirements" are going to be.

Many industry groups warn that regulating greenhouse gases under the Clean Air Act--as the EPA is doing--could harm the economy and create a cascade of litigation.

Although the agency has yet to publish an economic analysis of its greenhouse-gas proposals, the EPA is fighting the perception that Clean Air Act regulations will be burdensome.

"We're going to do it in a way that's first and foremost sensible," but also, "sensitive to economic concerns," McCarthy said. "We want to do it in a way that's deliberate, that's phased, that allows innovation to happen," she said.

The new rule won't come into force before January 2011, and then at first for only the largest emitters, such as coal-fired power plants. Other emitters will be phased in over time.

McCarthy said the agency wasn't going to require new technologies that hadn't been proven.

"The rules...look at moving forward already demonstrated technologies, not innovative technologies that have yet to be properly demonstrated," she said. For example, McCarthy said the rules "will not require carbon capture and storage at every facility."

One of the recommendations the EPA's likely to make for compliance with emission reductions is energy efficiency, she said. "To the extent to which we can guide companies to look at making investments that save their bottom line in a way that achieves their reduction goals, that's what we want to do," said McCarthy.

-By Ian Talley, Dow Jones Newswires; 202-862-9285; ian.talley@dowjones.com