Both President Obama and actor Leonardo DiCaprio pledged Tuesday to help protect the ocean and challenged other nations to undertake bold initiatives of their own before it's too late.

Speaking via video at a State Department conference, Obama stressed that the sea is more than an alluring landscape -- it's also a source of food and economic growth. Climate change, overfishing and pollution now threaten to degrade that resource, the president said. "We cannot afford to let that happen," he said. "That’s why the United States is leading the fight to protect our oceans."

Obama said he would use his executive authority "to protect some of our nation’s most precious marine landscapes." While the president did not specify what parts of the sea he would put off-limits, The Washington Post reported Tuesday that he plans to expand the Pacific Remote Islands National Marine Monument, a grouping of seven islands and atolls in the south-central Pacific Ocean.

"But for this effort to succeed, it has to be bigger than just one country," Obama cautioned.

Other global leaders gathered at the conference have also been pressing for stricter enforcement of maritime rules. The European Union has prohibited three countries -- Belize, Cambodia and Guinea -- from selling fish in its market because they engage in illegal fishing practices. And the EU has warned another eight nations that they could face a similar ban: Ghana, Curaçao, South Korea, Fiji, Panama, Sri Lanka, Togo and Vanuatu.

"There is a rush to the sea," said European fisheries commissioner Maria Damanaki in an interview. "We need leadership."

DiCaprio, for his part, said the Leonardo DiCaprio Foundation would spend $7 million over the next two years on marine conservation. He said the group would help support nations that are expanding marine reserves, as well as other initiatives. Noting that he had "witnessed environmental devastation firsthand" as he had gone diving in regions across the world, DiCaprio urged global leaders to be more ambitious.

"This isn't simply an exercise in wildlife conservation," he told the delegates assembled in Washington. "If we don't do something to save the ocean now, it won't be just the sharks and the dolphins that suffer. It will be our children and our grandchildren."

DiCaprio noted that his first charitable donation was to a group that protected endangered manatees in Florida, adding that he had revered the sea ever since he was young. "Before I wanted to become an actor, I dreamt of becoming a marine biologist," he said.

The advocacy group Oceana, which has received a $3 million grant from DiCaprio's foundation, praised Obama for also pledging to launch a federal initiative to crack down on illegal fishing. The group has estimated that from 20 percent to 32 percent of wild-caught seafood crossing U.S. borders is a product of from illegal, unregulated and underreported fishing.

"President Obama’s announcement is a historic step forward in the fight against seafood fraud and illegal fishing worldwide," said Beth Lowell, an Oceana campaign director. "This initiative is a practical solution to an ugly problem and will forever change the way we think about our seafood."

Some groups may resist aspects of Obama's proposal, however. Recreational fishing groups objected to the original designation of the Remote Pacific Islands National Marine Monument in 2009, and won an exemption for sportfishing activities. Mike Leonard, ocean resource policy director for the American Sportfishing Association, said recreational fishing enthusiasts would push to ensure their existing exemption stays in place if the protected area is expanded.

“We believe in almost all instances you can still have marine conservation and marine protection, and still allow for sustainable recreational fishing activities to take place,” Leonard said, adding there’s almost no sportfishing activity in the area because “It’s a heck of a trek out there. Our concern is obviously with the precedent this might set.”