U.S. Environmental Protection Agency Administrator Lisa Jackson had promised the rules proposal would be issued before the end of the year.
In a prepared statement, EPA did not offer a new timeline, but said the delay was for a "short period" and that the proposal would be issued "in the near future."
EPA cited the "complexity of the analysis" involved in the issue and said agency officials are "still actively clarifying and refining parts of the proposal."
"Administrator Jackson has been committed since the beginning of her administration to complete these efforts, and expects to issue a proposed rule in the near future," EPA said in its prepared statement.
EPA officials announced the delay just five days before the one-year anniversary of the December 22, 2008, collapse of a coal-ash impoundment at a Tennessee Valley Authority power plant in East Tennessee.
The spill at TVA's Kingston Plant sent more than 1 billion gallons of coal ash containing more than 2.6 million pounds of toxic pollutants pouring into local streams, fields and homes. The disaster gave new momentum to a long-standing campaign by environmental groups to more strictly regulate power plant ash.
Generally, coal-ash is not subject to any concrete set of national standards to govern the safety of the impoundments or toxic pollution that leaches from them. Rules vary widely from state to state.
For years, EPA was under congressional orders to decide if coal ash should be regulated as a hazardous waste. The Clinton administration moved to do so, but the Bush administration reversed the action.
In October, a U.S. Government Accountability Office report said the Obama administration was considering a "hybrid" approach that would regulate some coal-ash dumps as hazardous waste sites and subject others to less strict non-hazardous waste rules.
EPA's delay in issuing proposed rules also comes just after an industry group warned in congressional testimony that an EPA hazardous waste designation for coal ash could force nearly 200 coal-fired power plants nationwide to close.
"A national coal combustion products regulation will alter the technology and economics of coal-fired power plants," Ken Ladwig of the Electric Power Research Institute told a House subcommittee. "Some owners would decide to prematurely shut down rather than incur the costs of compliance, while others would convert their ash handling and disposal systems and continue to operate in the post-regulation market."
Earthjustice attorney Lisa Evans, a longtime advocate of tougher coal-ash regulations, called the EPA delay "unfortunate" and said she hopes that EPA "will prevail over the power industry's fear mongering and campaign of disinformation." Evans said many communities where coal-ash disposal sites are located "have been waiting decades for the protection that only EPA can deliver.
"It is impossible to imagine that the imposition of basic landfilling standards will bring down the U.S. power industry," Evans said. "It just won't happen."