The US is to establish what it calls "the largest area of protected sea in the world" around its Pacific islands.

Commercial fishing and mining will be banned in the protected zones which include the Marianas Trench, the deepest area of ocean on the planet.

The area totals 500,000 sq km (190,000 sq miles) of sea and sea floor.

While welcoming the protection package, environmental activists said that without curbing climate change, the other measures would be meaningless.

President George W Bush will formally announce the measure during an address on Tuesday evening in Washington.

Briefing journalists in advance, his environmental advisor James Connaughton said the move meant the US was "setting the mark for the world with respect to effective marine management".

"The conservation action is going to benefit the public and future generations through enhanced science, knowledge and awareness, and just good old-fashioned inspiration, because these places are exceptionally dynamic when it comes to the marine environment," said the chairman of the White House council on environmental quality.

The areas covered include some of the islands most remote from the world's large populations centres, which have not so far encountered the intense fishing present across much of the oceans.

They also encompass some of the most biologically diverse places on the planet, undersea volcanoes and hot seafloor vents, and submarine pools of sulphur thought to be unique on Earth.

War monuments

The measure involves establishing three new "national monuments" around different US territories in the Pacific.

Together they encompass the Marianas Trench and the long arc of volcanoes and undersea vents along the Mariana Islands chain, south of Japan and north of Papua New Guinea; coral reefs around the three northernmost islands of the Marianas; and eight more coral atolls and islands.

The Marianas group includes islands such as Saipan and Tinian which played significant roles in World War II, and Guam which is still a major US base.

One of the other places now receiving protection, Johnston Atoll, was formerly used to stockpile chemical weapons.

Mr. Connaughton said the national monuments would be established in a way "that also fully respects our nation's national security needs by ensuring freedom of navigation for all vessels in accordance with international law and by ensuring that our military can stay ready and be globally mobile".

The Marianas Trench, which reaches depths of about 11km (about seven miles), and the string of volcanoes and vents will be protected from mineral exploration.

The coral areas will also see a complete ban on commercial fishing out to 50 nautical miles from shore.

"It's very significant both from an ecological and biological perspective as well as in its political symbolism," said Joshua Reichert, managing director of the Pew Environment Group.

"In the Marianas alone, the area that's been protected contains some of world's most exceptional geology. Rose Atoll has the highest proportion of live coral cover anywhere in the world."

Brendan Cummings, oceans programme director at the Center for Biological Diversity which has brought several court actions against the Bush administration on climate change, also welcomed the commercial fishing ban but said curbing greenhouse emissions was also vital for the long-term preservation of corals.

"Unless we deal with global warming, all other protective measures for coral reefs will be rendered meaningless," he said.

"Ultimately, Bush's legacy as a climate criminal will far outweigh his ocean legacy, as any benefit coral reefs receive from this monument designation will be bleached away by warming seas."

As well as warming the oceans, rising carbon dioxide emissions are slowly reducing the alkalinity of seawater, which is also projected to have a detrimental effect on coral growth.

President Bush's administration has come under fire in recent months from environmentalists angered by its reluctance to cut carbon emissions, by its moves to weaken endangered species legislation and by its support for naval use of sonar systems that can kill whales.

But, said Mr Reichert, the outgoing president has "protected more special places in the sea than any other person in history".