The Obama administration announced Tuesday that it will clear a decades-long backlog of petitions for the endangered species list, agreeing to decide within six years whether 251 species deserve federal protection.

The settlement between the U.S. Fish and Wildlife Service and WildEarth Guardians is a partial truce in the long-running battle between left-leaning environmental groups and Fish and Wildlife Service officials, who say they lack the resources to add all the plants and animals that deserve protection to the federal endangered species list. It could pave the way for an avalanche of new listings under the Endangered Species Act, though it also limits the number of species that the wildlife group can petition to list.

“Managing the service’s listing program is a story of limited resources and an unlimited workload,” Gary Frazer, the agency’s assistant director for endangered species, said in an interview.

In recent years, the Fish and Wildlife Service has been inundated with lengthy petitions for listings from groups concerned that climate change, pollution and other forces that destroy habitats are moving more species to the brink of extinction. In fiscal 2010, the Fish and Wildlife Service spent so much of its $21 million listing budget on litigation and responding to petitions that it had almost no money to devote to placing new species under federal protection, according to agency officials.

At the moment, 1,374 U.S. species are listed under the Endangered Species Act. Once a plant or animal is listed as threatened or endangered with extinction, federal officials must designate critical habitat for it, which sometimes leads to clashes with local landowners or developers. The process also requires that any federal action — such as funding highway construction — that could affect a listed species undergo a review by Fish and Wildlife Service officials.

Deputy Interior Secretary David Hayes told reporters that the agreement is a breakthrough that will allow the department to rebalance its priorities.

“It’s clear that the status quo is simply unacceptable — not only for the service, but for the hundreds of imperiled species that deserve the service’s attention and resources,” Hayes said.

Under the agency’s work plan, which it filed in the U.S. District Court for the District of Columbia as part of its settlement, officials will decide whether to add to the endangered list 251 species that are now classified as “warranted but precluded” from the list.

Frazer emphasized that many of the 251 species will end up being listed. “It’s likely that the majority of the species on the 2010 list will be proposed for endangered species protection,” he said.

The number of imperiled species listed each year has seesawed over the past two decades: It averaged 65 under President Bill Clinton, eight under President George W. Bush and 25 under President Obama; most of those added under the current president have been Hawaiian species. In response, two groups — WildEarth Guardians and the Center for Biological Diversity — have filed dozens of petitions since 2007, aiming to force the listing of more than 1,230 plants and animals. That’s more than five times as many requests as in the 12 preceding years.

The Fish and Wildlife Service is also facing 20 court cases involving its handling of 320 species.

Under the settlement announced Tuesday, WildEarth Guardians can petition the government to list no more than 10 additional species each year.

“The service just needs to prioritize this backlog,” Nicole Rosmarino, wildlife program director for WildEarth Guardians, said in an interview. “There is just no shortage of imperiled animals and plants that really need the protection of the Endangered Species Act. . . . We think that waiting line needs to to start moving.”

Although Tuesday’s agreement appears to resolve the legal impasse that has dogged most of these species, it still needs to be accepted by the court, and it remains unclear whether the Center for Biological Diversity will sign off. The center petitioned on 250 of the species identified in the settlement. Kieran Suckling, who heads the group, said in an interview that his organization pulled out of talks with Interior Department officials “at the last hour” because the deal would have limited the number of petitions it could file and did not include key species such as the Pacific walrus and the American wolverine.

However, representatives of several other environmental groups, such as John Kostyack, vice president of wildlife conservation for the National Wildlife Federation, welcomed the settlement. “The focus should be getting species protected and avoiding unnecessary resources being devoted to the courtroom,” Kostyack said.

Last month, the Fish and Wildlife Service submitted a 2012 budget request to Congress in which it asked lawmakers to cap how many listing proposals it has to consider in a given year. “The many requests for species petitions has inundated the listing program’s domestic species listing capabilities,” the agency wrote.