WASHINGTON, May 2 (UPI) -- U.S. Secretary of Defense Chuck Hagel warned NATO members Friday to increase their defense expenditures in light of challenges posed by Russia.

Leaders across our governments must understand the consequences of lagging defense spending by the 28 member nations of NATO, he said in an address at the Wilson Center in Washington. Noting the disproportionate burden of defense spending by the United States, he added, Today Americas GDP (gross domestic product) is smaller than the combined GDPs of our 27 NATO allies, but Americas defense spending is three times our allies defense spending.

Hagels speech reinforced the Obama Administrations appeals to NATO nations to raise their defense spending. In March President Barack Obama urged every NATO member state to step up and carry its share of the burden by showing the political will to invest in our collective defense.

Obama has said only the strength of the alliance would push Russia toward a diplomatic solution in Ukraine, and NATO and U.S. officials have called for renewed emphasis on the NATO budget when its next summit is held in Wales in September.

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Hagel to NATO: increase defense spending