WASHINGTON — The Obama administration on Monday gave conditional approval to allow Shell to start drilling for oil off the Alaskan coast this summer, a major victory for the petroleum industry and a devastating blow to environmentalists.

The decision adds a complex new chapter to the legacy of President Obama, who has pursued the most ambitious environmental agenda of any president but has sought to balance those moves by opening up untouched federal waters to new oil and gas drilling.

Shell has sought for years to drill in the icy waters of the Chukchi Sea. Federal scientists believe the region could hold up to 15 billion barrels of oil.

The Interior Department decision angered environmentalists who for years have demanded that the administration reject offshore Arctic drilling proposals. They fear that a drilling accident in the treacherous Arctic Ocean waters could have far more devastating consequences than the deadly Gulf of Mexico spill of 2010, when the Deepwater Horizon rig explosion killed 11 men and sent millions of barrels of oil spewing into the water.