ANCHORAGE — A joint U.S.-Canada exploration of the Arctic is uncovering evidence that could boost the two countries' claims to lucrative oil and gas resources under the sea floor.

Since early August, icebreakers from the two countries have criss-crossed icy areas of the Beaufort Sea, measuring how far the continental shelf extends into the Arctic.

Nations routinely claim special rights within 200 nautical miles of their coasts, but their reach can go beyond that if they can prove to the United Nations that the continental shelf -- the underwater portion of the continent -- extends beyond that limit.

The farther the shelf extends, the more ocean floor that nations can claim: either exploiting or protecting it from development. The Arctic is thought to have billions of barrels worth of undiscovered oil and a third of the world's undiscovered gas, and the ice cap's retreat in recent years has heightened interest in both development and conservation.

The continental shelf ends quickly off Alaska's northeast coast but extends for hundreds of miles off Alaska's northwest and western coast. The shelf also extends off Canadian islands in the Arctic.

That means, in theory, that the United States and Canada could stake claims to a large swath of the Arctic sea floor, asserting exclusive control of minerals and creatures living there. Russia and Norway already have staked claims to extended areas of the Arctic, though the United Nations has not yet recognized Russia's claim.

For now, any claim the United States might stake to the continental shelf would lack international recognition because the United States has not ratified the international Law of the Sea treaty. Members of the Alaska congressional delegation have long supported ratification of the treaty.

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