US Defense Secretary Chuck Hagel on Friday renewed calls for members of the North Atlantic Treaty Organization (NATO) to increase defense spending, following Russia's recent action in Ukraine that "has reminded NATO of its founding purpose." "Although the NATO is to end its combat mission in Afghanistan later this year, challenges today remind the NATO members of "the enduring need for the alliance" and the need to strengthen it," the Pentagon chief was quoted as saying by Xinhua. "After more than a decade focused on counterinsurgency and stability operations, NATO must balance a newly-renewed emphasis on territorial defense with its unique expeditionary capabilities, because as we have seen, threats to the alliance neither start nor stop at Europe's doorstep," he said. Hagel said Russia's actions in Ukraine had ushered in "new realities" and made NATO's value "abundantly clear." "NATO must stand ready to visit the basic principles underlying its relationship with Russia," he added. He called on NATO members to increase their financial contributions to the alliance, noting that "the United States spends three times as much as the 27 other NATO members, which has been increasingly disproportionate within the alliance." "Over time, this lopsided burden threatens NATO's integrity, cohesion, and capability -- and, ultimately, both European and transatlantic security," he said, adding. "We must see renewed financial commitments from all NATO members. " In addition to investment in militaries, Hagel said the US and Europe must partner together to bolster energy security and blunt Russia's energy policies.