WASHINGTON: The United States on Thursday will join more than 20 countries to announce the creation of 40 new marine sanctuaries around the world to protect the world's oceans from the threat of climate change and pollution.

The sanctuaries, set to be unveiled at a high-level conference in Washington, will include the first such US monument in the Atlantic Ocean. The protected areas are meant to limit commercial development and human impacts on ocean ecosystems.

Altogether, countries attending the oceans conference will announce the addition of new sanctuaries covering nearly 460,000 square miles of ocean, an area around the size of the nation of South Africa.

President Barack Obama will address the conference on Thursday, where he will unveil the designation of a US marine monument off the coast of New England, the White House said.

The 4,913 square mile sanctuary, containing underwater mountains and canyons, will be the first national marine reserve in the Atlantic.

The move follows the administration's decision to expand a massive reserve off the coast of Hawaii last month, as Obama looks to cement his environmental legacy before his tenure ends next year.

Opponents of the new Atlantic monument have complained that it threatens the commercial seafood industry in the region. The administration said the reserve was designed to protect only the most vulnerable areas.

"We feel the approach is well justified and will help support a sustainable ecosystem over the long term," a senior administration official said.

The conference will emphasise the urgent need to combat illegal fishing, pollution and the acidification of the ocean, which is destroying coral reefs and shellfish, said Catherine Novelli, the US under secretary for economic growth, energy, and the environment at the State Department.

"We're expecting over a hundred new initiatives that are going to be worth billions of dollars," Novelli told reporters ahead of the conference at the US State Department.

Programmes to protect coastal areas and rid the seas of plastic debris are among the many initiatives to be announced.

Novelli said plastic debris was running into the ocean at such a pace that by 2050 there would be more plastic than fish in the sea.

US Secretary of State John Kerry, who will also address the conference on Thursday, has made climate change and protecting the ocean a priority during his term, traveling to the Arctic in June to highlight the impact of warming oceans on some of the world's largest glaciers in Norway and Greenland.