The United States joined forces with other nations Tuesday to declare a vast swath of the Pacific Ocean a marine sanctuary and take "historic" steps to combat illegal fishing.

Taking the helm in a stepped up fight to save the planet's oceans, President Barack Obama ordered his administration to chart a way to expand an existing US reserve and create what would be the world's largest marine park.

Combined with other pledges from nations such as the Cooks and Kiribati, the expansion of the Pacific Remote Islands National Monument would mean some three million square kilometers (1.2 million square miles) are declared off-limits to all fishing fleets and to drilling activities.

Some $1.4 billion was also pledged at a two-day conference to help fight pollution, acidification and overfishing endangering the health of the oceans.

"Right now only a small fraction of the world's ocean... is currently protected," US Secretary of State John Kerry said closing two days of landmark talks he hosted at the State Department.

More must be done, he said, but the new commitments are "a terrific start."

Leaders from more than 80 countries met for two days with scientists and industry experts to hammer out an action plan to protect the seas from man's encroachment.

Obama said he would use his executive powers to expand the Pacific Islands marine park, where the White House said "tropical coral reefs and associated marine ecosystems are among the most vulnerable areas to the impacts of climate change and ocean acidification."

In a video address to the conference, Obama said he had also directed the US government to "build a national strategy to combat black-market fishing."

"Rising levels of carbon dioxide are causing our oceans to acidify. Pollution endangers marine life. Overfishing threatens whole species as well as the people who depend on them for food and their livelihoods," Obama said.

Before the exact scope of the sanctuary is drawn up, "we will consider the input of fishermen, scientists, conservation experts, elected officials, and other stakeholders," the White House said.

- Bleached coral -

But the plan could ignite a new battle with Republicans in Congress, angered by Obama again using his executive powers to bypass the US legislature.

"If we ignore these problems, if we drain our oceans of the resources, we won't just be squandering one of the humanity's greatest treasures, we'll be cutting off one of the world's major sources of food and economic growth," Obama warned.

He won support from Hollywood actor Leonardo DiCaprio, an avid diver, who pledged his foundation would pump another $7 million over the next two years into projects to help the oceans.

"I've witnessed environmental devastation first hand," DiCaprio told the conference, recalling two dives he made some 18 years apart on the Coral Reef in Australia.

"What once had looked like an endless underwater utopia is now riddled with bleached coral reefs and massive dead zones."

"Unfortunately today, there's no proper law enforcement capacity and little accountability for violating the law. It's the Wild West on the high seas," warned the star of the box-office hit movie "Titanic."

Kerry said the move to clamp down on illegal fishing meant all seafood sold in the United States would be "sustainable and traceable."

A recent report found that between 20 percent and 32 percent of wild-caught seafood imported into the US in 2011 came from illegal or "pirate" fishing.

Environmentalists also welcomed Obama's plans as "a historic step forward in the fight against seafood fraud and illegal fishing worldwide."

"This initiative is a practical solution to an ugly problem and will forever change the way we think about our seafood," said Beth Lowell, campaign director with the international advocacy Oceana group.