The UN chief on Sunday announced an initiative to protect oceans from pollution and over-fishing and to combat rising sea levels which threaten hundreds of millions of the world's people.
United Nations Secretary-General Ban Ki-moon said the "Oceans Compact" initiative sets out a strategic vision for the UN system to more effectively tackle the "precarious state" of the world's seas.
Ban highlighted the "grave threat" from pollution, excessive fishing and global warming.
"Our oceans are heating and expanding," he said in a speech to a conference marking the 30th anniversary of the UN Convention on the Law of the Sea.
"We risk irrevocable changes in processes that we barely comprehend, such as the great currents that affect weather patterns.
"Ocean acidification (from absorbed carbon emissions) is eating into the very basis of our ocean life; and sea level rise threatens to re-draw the global map at the expense of hundreds of millions of the world's most vulnerable people."
The UN chief, who also called for action to curb piracy and irregular sea migration, said he hoped for progress towards a legally binding framework to combat "runaway climate change" at a UN conference in Doha in November.
But action could also be taken now.
Ban said the Compact was aimed at "improving the health of the oceans" and strengthening their management through an action plan to be overseen by a high-level advisory group.
This would be made up of senior policymakers, scientists and ocean experts, representatives from the private sector and civil society and leaders of the UN organisations involved.
The UN chief said his initiative would also support implementation of the Law of the Sea treaty, which came into force in 1994.
He called the treaty one of the world's "most significant legal instruments" and a tool for sustainable development which all nations should ratify.
"It contributes to international peace and security, the equitable and efficient use of ocean resources, the protection and preservation of the marine environment and the realisation of a just and equitable economic order."
The United States is the only major power not to have signed the convention. Republicans in the Senate contend it would undermine US sovereignty and are blocking ratification.
"The world's oceans are key to sustaining life on the planet," Ban said in his introduction to the Oceans Compact.
Among other objectives, the Compact aims to protect the world's people from ocean degradation and natural hazards such as tsunamis, from over-fishing and from pollution by land and sea activities.
It calls for countries most at risk from rising sea levels to develop plans to mitigate the threat, and for vulnerable regions to have tsunami warning systems.
By 2025, all countries should set national targets to curb nutrients, marine debris and wastewater.
The Compact calls for renewed efforts to curb illegal fishing, rebuild fish stocks and halt the spread of invasive alien species.
By 2020, it says, at least 10 percent of coastal and marine areas should be subject to conservation measures.