The Arctic may contain as much as a fifth of the world’s yet to-be-discovered oil and natural gas reserves, the United States Geological Survey said Wednesday as it unveiled the largest-ever survey of petroleum resources north of the Arctic Circle.

Oil companies have long suspected that the Arctic contained substantial energy resources, and have been spending billions recently to get their hands on tracts for exploration. As melting ice caps have opened up prospects that were once considered too harsh to explore, a race has begun among Arctic nations, including the United States, Russia, and Canada, for control of these resources.

The geological agency’s survey largely vindicates the rising interest. It suggests that most of the yet-to-be found resources are not under the North Pole but much closer to shore, in regions that are not subject to territorial dispute.

“For a variety of reasons, the possibility of oil and gas exploration in the Arctic has become much less hypothetical than it once was,” Donald L. Gautier, the chief geologist for the survey, said during a news conference Wednesday. “Most of the resources are on the continental shelf in areas already under territorial claims.”