WASHINGTON — One of Shell Oil’s two Arctic drilling rigs is beached on an island in the Gulf of Alaska, threatening environmental damage from a fuel spill and calling into question Shell’s plans to resume drilling in treacherous waters north of Alaska in the summer.

The condition of the rig, the Kulluk, which broke free from a tow ship in stormy seas and ran aground Monday night, was unknown late Tuesday. The Coast Guard was leading an effort to stabilize the vessel, 266 feet in diameter, to prevent further damage and keep more than 150,000 gallons of diesel fuel and lubricants from spilling onto the rocky shoreline.