SAVANNAH, Ga. (AP) — Conservation groups filed suit Wednesday against the federal government, saying it has taken too long to act on proposals to expand protections for critically endangered right whales.

The lawsuit in U.S. District Court in Washington says the National Marine Fisheries Service for more than eight years has ignored conservationists' petitions seeking greater speed limit restrictions on ships along the U.S. East Coast, where the rare whales' range extends from Maine to Florida.

Scientists say collisions with ships are one of the greatest threats to a species considered dangerously close to extinction. Last year, the National Oceanic and Atmospheric Administration estimated the population of North Atlantic right whales has dropped below 370. The previous year's estimate was 412.

Since 2008, the government has imposed speed limits on larger vessels in Atlantic waters in areas and periods of the year when right whales are frequently seen. The largest areas are off the coast of New England, where right whales feed and mate from spring into fall, and off the Carolinas, Georgia and northern Florida, where pregnant females migrate each winter to give birth.

The conservationists' lawsuit says the existing restrictions aren't good enough. Scientists have documented cases of 12 right whales being struck by vessels since 2013. Four were either killed or seriously injured — including a 6-month-old right whale calf found dead last year.

“The clock is running out for right whales, and further delay is unacceptable," Kristen Monsell, oceans legal director at the Center For Biological Diversity, said in a statement.

The center filed the lawsuit along with the groups Whale and Dolphin Conservation, Defenders of Wildlife and Conservation Law Foundation.

The conservationists are asking a federal judge to force the National Marine Fisheries Service to issue a decision on petitions they filed seeking expanded protections in 2012 and again last August.

They want speed limits imposed on vessels smaller than 65 feet (20 meters) long, which are currently exempt. They are also seeking expanded speed-limit zones in some areas and for reduced speeds to become mandatory in certain cases where they are now voluntary.

National Marine Fisheries Service spokeswoman Allison Garrett said the agency does not comment on pending litigation.