NAIROBI, Kenya (AP) -- The Obama administration has reversed years of U.S. policy by calling for a treaty to cut mercury pollution, which it described as the world's gravest chemical problem.

Some 6,000 tons of mercury enter the environment each year, about a third generated by power stations and coal fires. Much settles into the oceans where it enters the food chain and is concentrated in predatory fish like tuna.

Children and fetuses are particularly vulnerable to poisoning by the toxic metal, which can cause birth defects, brain damage and peeling skin.

Daniel Reifsnyder, the deputy assistant secretary of state for environment and sustainable development, told a global gathering of environmental ministers in Nairobi, Kenya, on Monday that the United States wants negotiations on limiting mercury to begin this year and conclude within three.

"We're prepared to help lead in developing a globally legally binding instrument," he said. "It is clear mercury is the most important global chemical issue facing us today that calls for immediate action."

The statement represented a "180-degree turnaround" from policy under the Bush administration, said Michael Bender, co-coordinator of the Zero Mercury Working Group, a global coalition of 75 environmental organizations working to reduce mercury exposure.

"The change is like night and day. The Bush administration opposed any international legal agreements on mercury and President (Barack) Obama is in office less than one month and is already supporting a global agreement," he said.

Bender said his group has had more discussions over mercury control in the past two weeks than they have in the last eight years and that the U.S. government included many of their ideas in the proposal they are presented in Nairobi.

Mercury is also widely used in chemical production and small-scale mining. The toxin can travel thousands of miles through the air or water.

The U.S. Food and Drug Administration, a regulatory agency, advises expectant mothers to limit weekly consumption to six ounces of albacore tuna or 12 ounces (340 grams) of "light" tuna, the health effects of which are still being scientifically debated. California authorities have been locked in a five-year legal battle to force tuna companies to paste warning labels on their product about potentially harmful mercury levels.

Despite the warnings, there's often little public knowledge of the dangers of mercury in seafood. In the American state of Idaho, a food bank distributed as much as 96 ounces (2,721 grams) of fish in family food baskets last summer. That's 48 times more than a child weighing less than 30 pounds (13 kilograms) is advised to eat monthly, according to the Health and Welfare advisory.

There is even less awareness in developing countries, where small-scale miners use mercury to pan for gold and fishermen eat contaminated fish or sell it to chic sushi restaurants.

"Murky? Maki?" asked Peter Omoga, manager at a Japanese restaurant in the Kenyan capital, when asked about mercury levels by an Associated Press correspondent tucking into a sushi feast.

While substitutes exist for almost all industrial processes that require mercury, more than 50 percent of mercury emissions come from coal-fueled power plants, complicating efforts to regulate it in countries that rely on coal for power.

A U.S.-drafted proposal obtained by The Associated Press would form a negotiating committee in conjunction with the U.N. environment program to help countries reduce their mercury use, clean up contaminated sites and find environmentally sound ways to store mercury. The European Union has already banned mercury exports starting in 2011. The U.S. has a similar ban that will be effective 2013, legislation that was sponsored by Obama when he was a U.S. senator.

Advocacy groups that have been working on getting such a global pact passed welcomed the U.S. policy change, saying it could encourage other countries such as Canada to make a similar change. Bender said mercury levels in the world had increased two to three times over the past 200 years.

"Given that the United States has pushed the door of resistance in a sense, that will lead others to follow," said Susan Egan Keane of the Washington, D.C.-based Natural Resources Defense Council.

Associated Press Writer Katharine Houreld in Nairobi, Kenya, contributed to this report.