At a UN oceans summit, delegates from China, Thailand, Indonesia and the Philippines said they would work to keep plastics out of the seas.
Some of the promises are not yet formalised and environmentalists say the measures proposed are not nearly urgent enough.
But UN officials praised the statement.
Meeting in New York, they said it was part of a clear international shift against ocean pollution.
Eric Solheim, the UN's environment director, told BBC News: "There are quite encouraging signs, with nations taking the ocean much more seriously. Of course, there is a very long way to go because the problems are huge."
It is estimated that 5-13 million tonnes of plastics flow into the world's oceans annually. Much of it is ingested by birds and fish â€“ and fragments of plastic have even been found in organisms at the bottom of the ocean.
A recent paper said much of the marine plastic often originates far from the sea â€“ especially in countries which have developed consumer economies faster than their ability to manage waste.
The  Helmholtz Centre in Leipzig, Germany, estimated that 75% of land-borne marine pollution comes from just 10 rivers, predominantly in Asia.
Reducing the plastic loads in these rivers by 50% would reduce global plastic inputs by 37%, it said.
Tom Dillon from the Pew Charitable Trusts, which campaign on oceans, urged China to move quickly.
He told BBC News: "For thousands of years the Maritime Silk Road was a pathway for export of Chinese culture and influence. Will the ocean be a vehicle for export of Chinese pollution, or a new culture of conservation and sustainability?"
A report to the UN conference from the Thailand government says most marine plastic debris is land-based, caused by inefficient waste management and poor handling of plastic wastes.
In Thailand, the total amount of garbage finding its way into the sea was estimated at 2.83 million tonnes in 2016 - of which 12% was plastic.
The Thai government says the nation has established a 20-year strategy to tackle the problem, including developing financial incentives for keeping plastic out of the sea and encouraging eco-packaging design and eco-friendly substitutes for plastics.
In Indonesia, the government is starting a mass education programme for schoolchildren, and in the Philippines new laws are being developed.
Part of the challenge is finding substitutes for plastics. An international prize for smarter materials and design for packaging was launched recently by the Ellen MacArthur Foundation.
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The company owners said the containers in Santos were full of acid and a disinfectant which came into contact with rainwater, causing a reaction.
The area's mayor said at least 66 people were taken to hospital with breathing difficulties.
Officials said the fire had been controlled but that there is still smoke in the area.
The cargo terminal and nearby homes were evacuated and residents were asked to stay inside.
The container terminal was operated by Localfrio, a logistics company, in Guaruja, an area on the eastern side of Santos, in Sao Paulo state.
A spokeswoman for the company, which exports chemicals used for refrigeration and general cargo, said the containers were filled with chloric acid and sodium dichloroisocyanurate - a cleaning and disinfectant agent.
Firefighters said rainwater had seeped into the containers causing a chemical reaction.
Local Mayor Mario Antonieta de Brito asked people to stay out of the rain which could "contain chemical elements that can burn the skin".
This is the latest in a series of recent incidents involving chemical companies in Brazil:
Nations responsible for much of the world's ocean plastic pollution have promised to start cleaning up their act.
[NEXT_CONCEPT]
A chemical explosion at a cargo warehouse in Brazil has spread toxic gas over the country's biggest port.