US President Barack Obama is planning to ban fishing, energy exploration and other activities in a large swath of the central Pacific Ocean, with Australia’s Great Barrier Reef given as an example of “environmental devastation”.

Obama’s proposal could create the world’s largest marine sanctuary.

US Secretary of State John Kerry, along with Hollywood star and environmental activist Leonardo DiCaprio, appeared at the Our Ocean Conference in Washington DC where a video message from the president was played.

“It’s fantastic to start off the day by hearing President Obama commit to expanding marine reserves in US waters and taking serious steps to prevent illegally caught fish from entering the marketplace,” DiCaprio said.

The actor pledged $US7 million ($7.4 million) for ocean conservation projects, in addition to a $US3 million donation last year.

“Since my very first dive in the Great Barrier Reef in Australia 20 years ago to the dive I got to do in the very same location just two years ago, I’ve witnessed environmental devastation first-hand,” DiCaprio said.

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

The proposed marine sanctuary would be expanded from about 225,000 square kilometres to more than two million square kilometres in a US-controlled Pacific Ocean area between Hawaii and American Samoa.

Atolls and islands in the area are mostly uninhabited.

“If we ignore these problems, if we drain our oceans of their resources, we won’t just be squandering one of humanity’s greatest treasures,” Obama said in a recorded video message.

“We’ll be cutting off one of the world’s major sources of food and economic growth, including for the United States. We cannot afford to let that happen.”