This story was updated at 2:40 p.m. EDT.

A coalition of states and environmental organizations is warning it might go back to court as soon as this month if a new agreement is not reached with U.S. EPA setting a timeline for new greenhouse gas rules for power plants.

The groups have sued to force EPA to propose New Source Performance Standards (NSPS) for electric utilities, which contribute more emissions linked to climate change than any other sector in the United States.

The plaintiffs, led by New York state, reminded EPA in a letter Friday that they have the right to resume litigation at any time now that EPA has failed to propose the NSPS rules by Sept. 30, as laid out in a settlement agreement. They added, however, that they would not "invoke their remedies" until at least Oct. 15, to allow all parties time to agree upon a revised schedule.

Environmental groups were frustrated last month when EPA announced it would miss the Sept. 30 deadline to propose the rules, the agency's second request for additional time since it and the plaintiffs reached a settlement agreement last December.

The agency has not said yet when it will propose the standards and whether it will also ask for additional time to finalize them. The December agreement called for the standards to be finalized by May 26, 2012.

Megan Ceronsky, an attorney with the Environmental Defense Fund, which is party to the suit, said negotiations with EPA were ongoing. She declined to comment on whether EPA had provided the plaintiffs with a new schedule for the rules.

"These emission standards are of critical importance as the power sector is the largest source of climate-destabilizing pollution in the United States," EDF said in a statement. "The standards are also long overdue -- EDF formally requested the Agency to take action to address the global warming pollution from the power sector six years ago."

EPA also declined to provide any details about the state of negotiations.

"EPA is continuing to work with petitioners on a new schedule for issuing the proposed greenhouse gas standards for power plants," the agency said in an email to Greenwire. "EPA has engaged in an extensive and open public process to gather the latest and best information prior to proposing [greenhouse gas] pollution standards for fossil fuel-fired power plants."

"We will fully consider all this information to develop smart, cost-effective and protective standards," EPA added.