WASHINGTON - President Obama moved to solidify his environmental legacy Tuesday by withdrawing hundreds of millions of acres of federally owned land in the Arctic and Atlantic Ocean from future offshore oil and gas drilling.

Obama used a little-known law called the Outer Continental Shelf Lands Act to protect large portions of the Chukchi and Beaufort Seas in the Arctic and a string of canyons in the Atlantic stretching from Massachusetts to Virginia from oil exploration and the potential for spills.

The announcement by the White House late in the afternoon was coordinated with similar steps being taken by Canadian Prime Minister Justin Trudeau to shield large areas of that nation's Arctic waters from drilling. Neither measure affects leases already held by oil and gas companies and drilling activity in state waters.

"These actions, and Canada's parallel actions, protect a sensitive and unique ecosystem that is unlike any other region on earth," the White House said in a statement. "They reflect the scientific assessment that, even with the high safety standards that both our countries have put in place, the risks of an oil spill in this region are significant and our ability to clean up from a spill in the region's harsh conditions is limited."

White House officials described their actions to make the areas off-limits to future oil and gas exploration and drilling as indefinite. Officials said the withdrawals under Section 12-A of the 1953 act used by presidents dating to Dwight Eisenhower cannot be undone by an incoming president. It is not clear if a Republican-controlled Congress can rescind Obama's action.

"There is a precedent of more than half a century of this authority being utilized by presidents of both parties," a White House aide said. "There is no authority for subsequent presidents to un-withdraw. ... I can't speak to what a future Congress will do."

"The U.S. is not acting alone today. Canada is acting to put an indefinite stop to activity in its waters as well," the aide said. "With Canada, we send a powerful signal and reinforce our commitment to work together."

U.S. and Canadian officials have negotiated for months to reach a joint understanding on how to manage adjacent areas in the ocean in an effort to make the new protections as sweeping and politically durable as possible. Meanwhile, advocacy groups lobbied Obama to ban oil and gas leasing in the Arctic entirely.

Obama already invoked the Outer Continental Shelf Lands Act to safeguard Alaska's Bristol Bay in 2014, and again last year to protect part of Alaska's Arctic coast. The president has protected 125 million acres in the region in the last two years, according to a White House fact sheet.

The Beaufort and Chukchi Seas are habitat for several species listed as endangered and species that are candidates for the endangered species, including the bowhead whale, fin whale, Pacific walrus and polar bear. Concern for the animals has heightened as the Arctic warms faster than anywhere else in the world and sea ice the bears use to hunt continues to melt.

The underwater canyons protected by the president cover nearly 4 million acres across the Atlantic continental shelf break, "running from Heezen Canyon offshore New England to Norfolk Canyon offshore the Chesapeake Bay," according to a separate fact sheet.

They are widely recognized as major biodiversity hot spots that are critical to fisheries. The canyons provide deep water corals used by a wide array of fish. The area also provides habitat "for ... deepwater corals, deep diving beaked whales, commercially valuable fishes, and significant numbers of habitat-forming soft and hard corals, sponges, and crabs," the White House said.

The American Petroleum Institute denounced the decision. "The administration's decision to remove key Arctic and Atlantic offshore areas from future leasing consideration ignores congressional intent, our national security, and vital, good-paying job opportunities for our shipyards, unions, and businesses of all types across the country," said Erik Milito, the group's Upstream director.

But a wide range of conservation groups hailed the decision. League of Conservation Voters President Gene Karpinski called it "an incredible holiday gift," saying that "an oil spill in these pristine waters would be devastating to the wildlife and people who live in the region."