NEW YORK - New York City urged New York state on Wednesday to ban natural gas drilling in its watershed, adding unprecedented support to critics who consider the chemicals used to mine for shale gas as poisonous to drinking water.

The biggest city in the United States joined environmentalists and small-town neighbors of drilling operations in trying to hinder the exploitation of one of the most promising sources of U.S. energy -- the Marcellus Shale formation.

The risks are simply not worth it, the city's acting Department of Environmental Protection commissioner, Steven Lawitts, said in a statement.

New York City's opposition marks the first time someone from Mayor Michael Bloomberg's administration has formally asked for a pre-emptive ban on gas drilling, and environmental groups said they believe it is the first time a U.S. city has sought such a prohibition for water safety reasons.

(Reporting by Edith Honan; editing by Daniel Trotta and Mohammad Zargham)