US Secretary of State John Kerry sounded the alarm Monday on the perils facing the world's oceans, calling for a global strategy to save the planet's life-giving seas.

"Let's develop a plan" to combat over-fishing, climate change and pollution, Kerry urged as he opened a two-day conference in Washington bringing together world leaders, scientists and industry captains.

"We as human beings share nothing so completely as the ocean that covers nearly three-quarters of our planet," the top US diplomat said.

Heads of government and state as well as ministers from some 80 countries gathered with researchers and experts from the fishing, plastics and farming industries at the State Department.

US President Barack Obama is expected to make a significant announcement about US conservation efforts during a video address on Tuesday.

And he will be followed by Hollywood A-lister Leonardo DiCaprio, whose foundation has pumped millions of dollars into efforts to protect the ocean habitat and marine species.

Setting the example on the first day, President Anote Tong of the low-lying Pacific nation of Kiribati announced that despite concerns about the economic fallout, all commercial fishing would be banned from January 2015 in the Phoenix Islands protected area.

"Addressing the challenges of climate change calls for very serious commitment and sacrifice," Tong said.

"The projected loss of revenue weighed very significantly in our consideration, but in the final analysis we made the decision to persist with effective sustainable strategies," he said to loud applause.

Environmentalists say the Phoenix Islands, one of the world's largest marine-protected areas rich in ocean life, provide shelter for such species as tunas and turtles, as well as reef fishes and sharks.

Kerry, long a passionate defender of the environment, warned there were already 500 "dead zones" around the world where marine life can no longer be sustained.

A third of the world's fish stocks are also "overexploited" and "nearly all the rest are being fished at or near their absolute maximum sustainable level."

If things continue without check, "a significant chunk of marine life may die out because it can no longer live... in the oceans water," Kerry said, warning of the risk of "breaking entire ecosystems."

- Vital security issue -

"No-one should mistake that the protection of our oceans is a vital security issue," Kerry insisted.

But he maintained that today's "ad-hoc approach... with each nation and community pursuing its own independent policy, simply will not suffice. That is not how the ocean works.

"We're not going to meet this challenge unless the community of nations comes together around a single comprehensive, global ocean strategy."

Kerry called for an "ambitious" program to reduce plastic trash sloshing around in the oceans, to better understand the effects of climate change on the acidification of the seas, and to cut agricultural nutrient run-offs.

"We cannot afford to put this global challenge on hold for another day. It's our ocean. It's our responsibility."

Twelve percent of the world's population depend on the oceans for their livelihoods.

Protecting the seas was also essential for food security, Kerry stressed, saying some three billion people "depend on fish as a significant source of protein."

Environmentalists have already identified three areas in US waters -- the remote northwestern Hawaiian islands, the Marianas and the Pacific atolls -- where existing marine parks could be vastly expanded.

"These are concerns that we share and we want to see put on the international agenda," Prince Albert of Monaco said as he met separately with Kerry.

"These issues concerning our global ocean don't concern only a few activists anymore -- it's the concern of all of us."