HALIFAX, Nova Scotia - The U.S. will assert its sovereignty in the Arctic, Defense Secretary Chuck Hagel warned Friday, even as Russia, China and other nations stake claims and expand their use of the icy waters for military exercises and transit.

Speaking at a security forum, Hagel said energy exploration in the largely untapped Arctic region could heighten international tensions, but that countries must work together to avoid conflict,

"We will remain prepared to detect, deter, prevent and defeat threats to our homeland and we will continue to exercise U.S. sovereignty in and around Alaska," Hagel said, as he unveiled the Pentagon's new Arctic strategy.

With a nod to the increased interest in the Arctic's lucrative oil and gas deposits, he added: "Throughout human history, mankind has raced to discover the next frontier. And time after time, discovery was swiftly followed by conflict. We cannot erase this history. But we can assure that history does not repeat itself in the Arctic."

Hagel's comments came as the military finalized plans to expand operations in the vast waters of the Arctic, where melting ice caps are opening sea lanes and giving nations like Russia greater access to the oil and gas deposits.

But it will take money and resources for the U.S. to fill the wide gaps in satellite and communications coverage, add deep-water ports and buy more ships that can withstand the frigid waters or break through the ice.

Hagel acknowledged the budget pressures, but he said the U.S. must map out its long-range plans despite the ongoing "deep and abrupt" spending cuts.

There are no cost or budget estimates yet. But by the end of this year, the Navy will complete plans that lay out what the U.S. needs to do to increase communications, harden ships and negotiate international agreements so nations will be able to track traffic in the Arctic and conduct search-and-rescue missions.

In his speech, Hagel said the U.S. will remain prepared to defend itself from threats in the region, preserve freedom of transit across the seas, plan for gradual upgrades to the fleet, improve mapping and understanding of the environment, and expand military ties with other Arctic nations.

He also said the U.S. will be ready to respond to any natural or man-made disasters in the Arctic, and will work with other nations and groups to protect the fragile environment.

President Barack Obama in May unveiled a 13-page U.S. strategy for the Arctic, asserting that nations must protect the region's fragile environment and keep it free from conflict. At the same time, however, the U.S. wants to make sure it is not left behind as countries like Russia, China, Canada and Norway map out plans ranging from gas and oil exploration to research and military exercises.

U.S. officials estimate the Arctic holds 13 percent of the world's undiscovered oil reserves and 30 percent of undiscovered gas deposits. Until recently, however, areas that could reap hundreds of billions of dollars in revenues were frozen over and unreachable.

Experts now say the Arctic waters could see largely ice-free summers as early as 2030, and there could be ice-free conditions for as long as a month by the mid-2020s. Ice-free means that about 10 percent of the water is ice-covered.