Far off America's shores, an ocean preserve flush with rare seabirds, turtles and marine mammals will grow to roughly three times the size of California under a memorandum that President Barack Obama signed Thursday.

The expanded Pacific Remote Islands Marine National Monument will cover 1.2-million square kilometres, making it the largest marine preserve in the world, the White House said. The move puts the remote waters surrounding a collection of islands off-limits to drilling and most fishing in a bid to protect fragile underwater life.

"This really is a matter of stewardship. It's also a matter of generational responsibility," U.S. Secretary of State John Kerry said Thursday. "We have a responsibility to make sure our kids and their families and the future has the same ocean to serve it in the same way as we have — not to be abused, but to preserve and utilize."

Millions of marine animals live in the bio-rich expanse included by the new monument, which will also add new protections for more than 130 "seamounts" — underwater mountains where rare or undiscovered species are frequently found. Commercial fishing, deep-sea mining and other extraction of underwater resources will be banned, but recreational fishing will still be allowed, in an attempt to preserve the public's access to federal areas.

Plan falls short of ultimate goal

The move to broaden the George W. Bush-era preserve comes as Obama seeks to show concrete presidential action to protect the environment, despite firm opposition in Congress to new environmental legislation. At the United Nations this week, Obama announced new U.S. commitments to help other nations deal with the effects of climate change, as world leaders seek to galvanize support for a major global climate treaty to be finalized next year in Paris.

Yet the expansion falls far short of what Obama could have done in the Pacific had he used the full extent of his powers.

Maritime law gives the U.S. control up to 200 nautical miles (about 370 kilometres) from the coast. Under Bush, the U.S. set aside waters extending about 80 kilometres from the shore of the remote, U.S.-administered islands in the south-central Pacific, thousands of miles from the American mainland.

The islands sit between Hawaii and American Samoa and are divided into five regions. Obama is extending the preserve to the full 320 kilometres— but only for three of the five regions.

Had Obama expanded the preserve in all five regions, he could have protected more than 2 million square kilometres, according to a geographic analysis by the Pew Charitable Trusts.

'Pristine and undisturbed'

Still, environmental groups cheered the announcement and said they hoped it would spur other nations to take similar steps to preserve the world's oceans.

"The president acted expeditiously, while the area is still largely pristine and undisturbed," said Frances Beinecke, president of the Natural Resources Defense Council.

Obama signed the memorandum on Thursday, the White House said, shortly before the president returns to Washington after three days of meetings with world leaders at the UN General Assembly. Obama first signalled his intent to expand the monument in June and asked for input on the final boundaries from fishermen, lawmakers and scientists. Officials said they received more than 170,000 electronic comments on the proposal.

While a major symbolic victory for environmentalists, who long urged Obama to take this step, the designation will have limited practical implications. That's because little fishing or drilling are taking place in the region even without the new protections.